GM 54952 EVALUATION REPORT, MANICOUAGAN PROPERTY 2N /

MINERAUX MANIC INC.

EVALUATION REPORT rri

MANICOUAGAN PROPERTY

MANICOUAGAN AREA, QUÉBEC NTS 22N/07

Prepared by

Luciano Vendittelli, B.Sc., APGGQ

MRN - GÉOINFORMATION 1997 GM 54952

June 1997 Montreal, Qc

Gouvernement du Québec Ministère de l'Energle et des Ressources ® Direction générale de l'Exploration géologique et minérale

CARTE MINÉRALE DU QUÉBEC, CANADA MINERAL MAP OF QUEBEC, CANADA

• Cr p Amazonite An Anorthosite ® Cu A Bruche Ca Calcaire m Cu,Ni (Pt,Pd) ® Chrysotile (1) Gr Granitoides p Dolomite • Fe Quagtaq Feldspath Tourbe • Fell A Graphite O LI ® Halite (2) O Mo p Magnésite Baie d'Ungava O NI p Néphéline m NI,Cu (Pt,Pd) p Olivine O Au A Phlogopite (3) O U p Pyrite, Pyrrhotite ® Zn A Pyrochiore Pyrophyllite m Quartz (4) P Talc Baie d'Hudson p Wollastonite

(1) Amiante (2) Sel gemme (3) Mica (4) Silice

Baie Jamps,A

lie d'Anticosti

Golfe du Saint-Laurent

lies de la `, Madeleine

NOUVEAU- I I Roches sédimentaires BRUNSWICK L—) Intrusions telsiques et intermédiaires Roches mafiques et ultramafiques ONTARIO Intrusions et gneiss chamockitiques SFjertir• ~oélke 7-1 r Complexe gneissique ÉTATS-UNIS Métasédiments I I • I 1 Roches volcaniques (laves, tuts) Sites météoritiques 4 190 290 Kilomètres Capitale provinciale

Représentation simplifiée de la carte originale é l'échelle de 1: 1 500 000 Centre de diffusion 5700, 4e Avenue ouest, local A-201 FIGURE 1 Chadesbourg (Québec) G1H 6R1 Téléphone: (418) 643.4601 Od ~ Télécopieur: (418) 644-3814 Québec (,7® Compilé par L. Avramtchev PRO 93-06 Service d'Information et de soutien è l'exploration minière (Remplace le PRO 87-01) TABLE OF CONTENTS

SUMMARY 4

1.0 INTRODUCTION 5

_ 2.0 PROPERTY DESCRIPTION 6

3.0 REGIONAL GEOLOGY 9

4.0 PROPERTY GEOLOGY 11

5.0 PREVIOUS WORK 14

6.0 DISCUSSION 24

7.0 CONCLUSIONS 26

-- 8.0 RECOMMENDATIONS 28

9.0 REFERENCES 32

ANNEXES CERTIFICATE OF QUALIFICATION 34 SUPPLEMENTARY INFORMATION AND CORE LOG 35

FIGURES LOCATION MAP 2 CLAIMS MAP 8 REGIONAL GEOLOGY 10 SUMMARY

This report has been commissioned by Mineraux Manic Mining Inc., Montréal, Québec, as a part of an on-going investigation into the base and precious-metal mining potential of the company's Manicouagan, Québec property.

The present study involves the collection and summary of the available material on the Manicouagan structure, and also some considerations on possible models of mineralization that could explain the magnetic anomaly at the center of the crater.

Authors lend credit to several orogenetic models where mineralization is related to the impact of a meteorite, combined with post-impact magmatism and hydrothermal remobilization of elements. Isotopic studies indicate that the impact occurred 2.1- 2.2 hundred million years ago.

I would like to thank Dr. Richard A.F. Grieve of the Geological Survey of Canada for his useful comments and Roger Moar and Marlene MacKinnon for their help in the preparation of this report. 1.0 INTRODUCTION

The Manicouagan structure is located in the central part of the Province of Québec at 51°25' N and 68°45' W (Fig. 1).

Access to the property is possible by the all-season highway 389 to a small air base near the Manic Five power dam; or to the Relais Gabriel on the east side of the crater followed by a fifteen minutes float plane or helicopter flight to the base camp at Lac des Isles.

SOUTH-WEST MANICOUAGAN PROPERTY

INTRODUCTION 5 ~ ~ ,© : ~;. ,,

2.0 PROPERTY DESCRIPTION

Minéraux Manic holds 216 claims in the geographic center of Ile René-Levasseur in the Manicouagan crater, approximately 300 kilometers North of Baie-Comeau. The claims are grouped in two blocks, one of 54 contiguous claims and another block of 162 contiguous claims. Each claim covers 16 hectares for a total of 4,391 Ha. The two blocks of claims are 2,400 meters apart.

Expiry Date: 04 OCT 1997

Claim Numbers: TOTAL: 216

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MANICOUAGAN PROPERTY

PROPERTY DESCRIPTION 6 5037620 5037621 5037622 5037623 5037624 5037625 5037626 5037627 5037628 5037629 5037630 5037631 5037632 5037633 5037634 5037635 5037636 5037637 5037638 5037639 5037640 5037641 5037642 5037643 5037644 5037645 5037646 5037647 5037648 5037649 5037650 5037651 5037652 5037653 5037654 5037655 5037656 5037657 5037658 5037659 5037660 5037661 5037662 5037663 5037664 5037665 5037666 5037667 5037668 5037669 5037670 5037671 5037672 5037673 5037674 5037675 5037676 5037677 5037678 5037679 5037680 5037681 5037682 5037683 5037684 5037685 5037686 5037687 5037688 5037689

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MANICOUAGAN PROPERTY

PROPERTY DESCRIPTION 7

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3.0 REGIONAL GEOLOGY

The property is located within the Grenville structural province of the Canadian shield. On a regional scale the rocks are metamorphosed to upper amphibolite and locally granulite facies trending northeast-southwest. This structural fabric is interrupted by the Cretaceous-Tertiary meteor impact structure which created a unique suite of rocks.

Physically, the impact structure is a 100 km circular basin with a central uplift of anorthositic composition (Mont de Babel) believed to be due to lithostatic rebound. The structure is evident on Landsat photographs (Fig. 2). These anorthosites are surrounded by melt rock, rimmed by a margin of latite and suevite (fall back breccia). The ring of water was formed when two narrow crescent shaped rivers, Manicouagan and Mouchalagane, and surrounding incised lands were flooded in 1974 by Hydro-Québec dam Manic 5, to form the Manicouagan Reservoir.

MANICOUAGAN PROPERTY

REGIONAL GEOLOGY 9 N

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Melt rocks' Anorthcsite Mafic gneiss Grey gneiss complex ~%~ Mixed gneiss _ ,

Undif f erentiated granitic gneiss' F , _ ' ~~ -Gabbro

Approximate center of structure km ;p l ////'/.////////, Fie. 2. Simplified geolocic map of the Manicouagan structure. Modified from Grieve and Horan (1978). .. .,,.. ., ~. ~.=r ~. • .. ... ~.~ .. ::~~...... ,.~ ~.. .MAINE

4.0 PROPERTY GEOLOGY

The circular structure is considered to be the product of an hypervelocity-meteorite impact with the Earth (astrobleme). Outcrop is controlled by topography. The main lithology of the Manicouagan structure is a flat-lying sheet of clast bearing impact melt (Manicouagan melt rock) some 100 to 200 meters thick and 55 kilometers in diameter which is found within the central part of the structure (Fig. 3). The original thickness of this sheet may have been up to 400 meters. Plagioclase, sanidine and augite are the main minerals present in the matrix of the melted rocks while hypersthene, quartz, iron oxides and smectite are minor components.

The Manicouagan melt rocks have been petrologically subdivided by Floran et al. (1978) into a lower, middle and upper unit.

4.1 MANICOUAGAN MELT ROCKS

The lower unit has a pseudoporphyritic texture in which abundant clasts of plagioclase, quartz and anorthosite reside in a very fine-grained matrix. At the base this unit is in unconformable contact with an undulating basement of Precambrian Grenville rocks. Here clasts/blocks up to 50 meters in size are noted.

The middle unit is characterized by a fine-grained, clast-bearing matrix and a poikilitic texture. This texture is described as being reminiscent of some lunar impact-melt rocks.

The upper unit is observed to be relatively coarse grained, but comparatively clast-poor. These units are overlain by suevite. Finally, suevite forms a very heterogeneous unit, consisting of 35% subangular partially digested gneiss fragments in a dark grey vesicular (10%) groundmass. The fragments vary greatly in size from a few millimeters up to 10 centimeters. The groundmass is siliceous with 5% felsic laths (up to 2 mm) and 2% angular glass-like shards.

4.2 PRECAMBRIAN BASEMENT ROCKS

Precambrian rocks include anorthosite, gabbro and a variety of gneissic rocks. Gneissic rocks can be grouped into quartz-feldspar (grey) gneiss and a series of mafic, charnokitic and transitional

MANICOUAGAN PROPERTY

PROPERTY GEOLOGY 11 gneisses. The anorthosites occupy a prominent uplifted block within the center of the Manicouagan structure.

Remains of sedimentary rocks of Paleozoic-age may occur locally on a regional scale and can be found as rare inclusions within the melt. This could provide the source of carbon for the formation of VD or normal diamonds in the area.

In an inward traverse from the outer limits of the impact zone toward the center of the crater, the intensity of the impact fracturing increases within the paragneiss basement suite. It commences with three moderately spaced (10 to 15cm) planes of brittle fracture, resulting in blocky outcrops, and within a space of less than ten meters the blockiness increases significantly due to narrowly spaced fractures and occasional narrow shears.

4.3 IMPACT-AFFECTED BASEMENT ROCKS

The basement suite grades into a brecciated gneiss with a continued increase in brittle deformation. These outcrops are characterized by one half to one meter of saprolite covering a crumbly oxidized surface. This breccia is crosscut by narrow continuous steeply dipping shears subparallel to the gneissosity at 110° to 125°. Sulphide mineralization accompanies this brittle-ductile deformation pattern. Pyrrhotite and up to 3% of pyrite can occur as discontinuous drusy stringers along fracture and shear planes within a groundmass of crushed gneiss and "rubbleized rock" (field term).

Along several fractures on the rock one can find traces to 2% disseminated fine grained pyrite. Interestingly, this sulphide mineralization, though minor, is also found plating late brittle fractures, which can be interpreted as postmetamorphic sulphide mineralization.

Another significant unit within the basement suite is the brecciated mafic gneiss. This unit's relative position with respect to the banded gneiss is uncertain, but from preliminary field observations (Kenwood, 1991) it likely lies between the brecciated gneiss and the suevite. This unit is chiefly composed of weakly foliated pyroxene-rich rounded fragments in a very fine grained black matrix. The mafic ragments vary in size from 0.5 cm to 20 cm, and are often slightly elliptical or tear drop shaped with their long axis oriented downdip. The matrix is aphanitic and locally contains dull black tachylite veinlets. Mineralization is irregular, with traces of pyrite (up to 2%), magnetite and probably illmenite.

MANICOUAGAN PROPERTY

PROPERTY GEOLOGY 12 4.4 POST-IMPACT INTRUSIVES

The post-impact ultramafic dykes are fresh, undeformed and contain no metamorphic minerals such is typical of the Grenville region (e.g. garnet). Unlike surrounding rocks, there is also no evidence of shock-metamorphism due to the meteoritic impact. All rocks in this area of similar composition and subjected to the impact would have been transformed into magnetite-bearing units. This is interpreted as being evidence for post-impact emplacement (Boivin, 1994). These dykes are fresh, unmetamorphosed and unaltered coarse-grained peridotite-pyroxenite. Owing to their ultramafic composition and apparent lack of zircons, age-dating these dykes will be difficult. The petrographic examination of a sample collected in 1990 reveals that the rock is a plagioclase bearing pyroxene hornblendite consisting of 90% amphiboles and pyroxenes and 5% plagioclase, 3% iron bearing biotite and 1-2% opaques (Chabot, 1990).

MANICOUAGAN PROPERTY

PROPERTY GEOLOGY 13 5.0 PREVIOUS WORK

5.1 1972 STUDY OF THE GEOLOGY AND PETROLOGY OF THE MANICOUAGAN RESURGENT CALDERA.

Currie interpreted the Manicouagan structure as a resurgent caldera and proposed a volcanic-tectonic model as its mode of origin. In this model, Currie proposes the updoming of an area larger than the present structure followed by radial explosions along marginal radial fractures then by explosions in voids in the interior of the structure. He interprets these explosions as having been chemical in nature caused by detonation of water-poor, hydrogen-rich gas emanating from a magma at depth. Such detonations everywhere caused the shock metamorphism and, in voids in the fractured center of the dome, produced the shatter cones. Degassing caused the collapse of the dome. This collapse was followed by the intrusion and eruption of alkali basalt and intrusion of larvikite (Murtaugh's monzonite) and at the base of this, trachyandesite (Murtaugh's latite). Currie interprets the pseudotachylite as a mixture of droplets of magma mixed with rock powder from the walls of the fractures. Currie proposes Mont de Babel rose on subterranean igneous rocks. This was followed by normal faulting which downdropped parts of the Manicouagan Complex.

This theory has since fallen into disfavor with the subsequent evidence and scientific evolution of knowledge of the Sudbury and other impact features. Murtaugh himself converted with the times as his subsequent work indicates. As one of the few existing references on the impact site at the time, it is not surprising that elements of Currie's musing with respect to magnetization and structural events were constructed and built upon by later workers who adopted some of the ideas and applied them to geophysical models and studies written by geophysicists.

5.2 1976 GEOLOGY OF THE MANICOUAGAN IMPACT STRUCTURE

Murtaugh (1976) fully mapped and described the geology of the Manicouagan structure and proposed an alternate model for its origin. He proposes that a hypervelocity impact of a cosmic body formed the structure. According to him, the breccias of the Manicouagan Complex were formed by or were related to the impact melt. Murtaugh also states that the igneous rocks of the Manicouagan Complex may be of endogenous origin, but that field relations suggest they were impact melt.

Murtaugh classified the rocks of the Manicouagan Complex into four categories: shock metamorphosed rocks, breccias, igneous rocks and contact metamorphosed country rocks. He also classified the shock metamorphosed rocks into six shock stages based on the extent of shock effects observed.

Based on the presence of diaplectic glass, Murtaugh determined the rocks were shocked to pressures of 350 kb which are beyond the pressures generated by known volcanic explosions. Also, shock

MANICOUAGAN PROPERTY

PREVIOUS WORK 14

metamorphism in the country rocks indicates that Manicouagan is an impact structure. He suggests it is unlikely a magma could produce, mix, disperse and quench the different types of heterogeneous glasses observed.

Local suevite selvages on basalt suggest that the two rocks may have been formed contemporaneously. Murtaugh describes the ellipsoids and lenticles of glass, devitrified glass or recrystallized material in tachylite at Manicouagan as similar to the spheroids from the West Clearwater Lake impact crater. He interprets the basalt as being impact melt from a totally vaporized or fused part of a turbulent cloud of volatilized, fused and fragmented country rock that deposited the suevite.

Murtaugh points out that the contact metamorphic aureoles around some tabular bodies of igneous rock are of extraordinary widths. In order to account for the observed contact metamorphic effects, he proposes the country rocks were preheated to high temperatures, as evidenced by a thin vein of pseudotachylite bordered by altered hypersthene and vitrified scapolite. The pseudotachylite alone could not have contained sufficient heat to have caused vitrification of scapolite and then cooled to a glass. Murtaugh proposes the source of preheating was the residual shock heat from the hypervelocity impact of an extraterrestrial body. He suggests that the rocks near the surface of the primary crater would be intruded and overlain by impact melt before the residual shock heat could dissipate.

The hills around Mont de Babel and in its meridian valley are interpreted by Murtaugh as blocks that became detached from the mountain at the time of uplift. He interprets the observation that rocks at the summit of Mont de Babel show decomposition of mafic minerals only near veins of pseudotachylite as suggesting that Mont de Babel rose above the main body of the impact melt. Zeolitization of the anorthosite on the mountain suggests Mont de Babel was at one time covered by fallback melt.

5.3 1978 CENTRAL MAGNETIC ANOMALY STUDY

The basis for this study by Coles and Clark (1978) was a 1969 GSC aeromagnetic survey flown at spacings close to a mile apart which yielded the signature of a wide elliptical magnetic anomaly. Coles and Clark concluded that highly magnetic rocks occur close to the surface over an area of about 8 x 12 km with a depth to the base of the magnetic body at not more than 3 km. They suggested that the cause of the anomaly could be the impact-generated magnetization of a mafic body which was subsequently uplifted.

From systematic sampling and measuring of the magnetic susceptibility of the country and melt rocks, Coles and Clark concluded that none of the samples, with the exception of two shocked ultra- mafic rocks, is sufficiently magnetic to form any significant portion of the causative body of the central anomaly.

MANICOUAGAN PROPERTY

PREVIOUS WORK 15 Coles and Clark discuss the cause of the proposed mafic or ultra-mafic body perhaps underlying the central anomaly. They suggest it is unlikely that thermal magnetization could have caused the remanent magnetization as there is little evidence that the temperature below the melt rock would be as high as the Curie temperature over a large enough volume. They believe that a shock pressure- induced magnetization is adequate to produce the magnetization of the mafic minerals, as such an impact can produce an order of magnetization of 20 to 30 GPa, which decreases rapidly downward and outward. Coles and Clark are of the opinion that the steep vertical gradient of the central magnetic anomaly can be explained by the rapid decrease in pressure downward and outward and would be further delineated by rapid uplift of the affected area.

With respect to the Coles and Clark interpretation, L. Boivin (1994) concludes that the mafic body which would have been magnetized would have had to be the only plug-like mafic geological unit in a rather large region and that these rocks would have had to be ultra-mafic in order to produce 100% magnetite equivalent. To her knowledge, there are no ultra-mafic units of that size in this Grenville region, although there are occurrences of mafic gabbroic gneisses in the NE region of the crater. These mafic gabbroic gneisses do not demonstrate the high magnetization gradient of the _ central rocks, despite the fact that virtually all mafic minerals within the confines of the crater have been transformed to magnetite. The central rocks could not have produced a higher magnetization than other rocks of similar composition especially when created by the same event at the same geological time.

5.4 1978 CHEMICAL STUDIES OF THE MANICOUAGAN IMPACT MELT

5.4.1 A. Chemical interpretation of the Manicouagan impact melt by Grieve and Floran

The Triassic melt rocks at the Manicouagan structure have been interpreted both as volcanic (Currie, 1972) and as the product of impact melting (Dense, 1971; Murtaugh, 1976; Floran et al, 1976). The data base for this study consists of earlier major element data from the GSC study (Currie, 1972) and major and trace element analysis performed at the Johnson Space Center in Houston (Floran et al 1976, 1978). In this study, Grieve and Floran favor the impact melting interpretation and present chemical arguments compatible with the impact origin. They comment on the composition of the melt and its relationship to the underlying basement units. In addition, Grieve and Floran used the chemical and petrographic data to constrain a model for the formation of the melt and the Manicouagan structure in a hypervelocity impact event.

Grieve and Floran conclude that the melt sheet is compositionally homogeneous in relation to the underlying and surrounding basement rocks. They have successfully modeled the melt for both major and trace elements as a mixture of chemically distinct lithologies, a property considered characteristic of impact melts. The mixture of diverse target lithologies to form a generally homogeneous melt composition is a result of the dynamic conditions accompanying melt genesis.

MANICOUAGAN PROPERTY

PREVIOUS WORK 16 Their model for the generation of the Manicouagan structure and melt sheet in a hypervelocity impact event proposes that compositional homogeneity results from the melted portion of the target containing internal velocity gradients of several kilometers per second and the melt being driven into the expanding cavity as a high velocity turbulent flow of superheated silicate liquid. During movement the silicate liquid incorporates crystalline inclusions the number and type of which are a function of the path traveled by the melt. In general, the number of inclusions decreases upward and toward the center of the structure establishing a lateral and vertical stratigraphy in an otherwise coherent melt sheet. These internal relationships derived from the cratering model are consistent with the detailed stratigraphy of the melt sheet described by Floran et al, 1978.

5.4.2 1978 Study of the stratigraphy, petrology and chemistry of the Manicouagan impact melt by Floran et al

Floran et al (1978) studied a sheet of clast-laden impact melt that forms an annular plateau surrounding an uplift of shocked anorthosite. The researchers analyzed 24 representative melt rocks. Based on internal stratigraphy, petrology and major and trace element geochemistry, they present a three-dimensional model for the crystallization history of the Manicouagan melt rocks.

Floran et al determined the bulk composition of the melt rocks to resemble that of terrestrial andesites and monzonites. They also found chemical homogeneity to be a major characteristic of the Manicouagan impact melt and suggest that homogenization was achieved early in the melt's history. They attribute the undifferentiated nature of the impact melt to the extreme sheetlike form of the melt body and to the viscosity of the clast-laden melt during crystallization. These researchers also found that textural heterogeneity is a major feature of the Manicouagan impact site. This textural diversity in the melt sheet reflects variation in pyroxene and feldspar morphology, matrix grain size and clast content. Superimposed on the dominant chemical and textural features of the melt are second-order effects leading to chemical heterogeneity on a local scale and textural homogeneity on a limited regional scale.

Based on decreasing clast abundance and on coarsening of the melt above the base, Floran et al divide the melt sheet into three vertically gradational units; a lower unit, a middle unit and an upper unit. The mineralogy of the three units is similar. The upper unit contains inverted pigeonite and pseudomorphs after olivine while the middle and lower units contain rare biotite and hornblende.

The lower unit is very fine-grained, clast-rich and dominated by a pseudoporphyritic texture. This unit contains a local facies called Subunit A, which is found only as dykes and pods of melt rock adjacent to large basement rocks or inclusions. This subunit forms a spherulitic to basaltic textural sequence with skeletal crystal morphologies indicative of extreme undercooling.

The middle unit is fine-grained, clast-rich and consists of a variety of textural types including microophitic, poikilitic and transitional. Development of the poikilitic texture within this unit can be modeled by a two stage cooling history in which the mafic composition of the melt permitted augite to be the liquidus phase. Supercooling caused by a high clast content initiated nucleation of

MANICOUAGAN PROPERTY

PREVIOUS WORK 17

augite adjacent to plagioclase clasts which then preferentially nucleated on relict clasts and enveloped earlier formed augite during growth. A similar model is applicable to some of the aluminous Apollo 17 poikilitic-textured impact melt rocks.

The upper unit is medium-grained and clast-poor with a hypidiomorphic granular texture. Textural homogeneity is most typical of this unit. It is similar in grain size and texture to hypabyssal intrusions. The continuous nature of this unit and its consistent texture indicate a more uniform and slower cooling history than that experienced by the bulk of the clast laden melt.

All three melt units are variable in thickness with the coarser-grained middle and upper units becoming more prominent closer to the center of the structure. According to Floran et al, this suggests the melt cooled more slowly in the central region, possibly due to the high post-shock residual temperature of the basement and included clasts.

5.5 1990 AERODAT SURVEY

This survey was flown by helicopter at 200 m. spacings. Subsequently, Aerodat produced a derivative gradient map of the central anomaly. This survey was done with hopes of providing the means for an interpretation of the depth to the summit of the magnetic body. The results obtained from this survey do not give this information but the survey confirms and enhances the previous GSC survey of the area.

Aerodat interprets the form of the magnetic anomaly as conforming to the response from a thick plate, slightly dish shaped along its top surface. They state that the upper surface of a large intrusive body would create a similar response but would require the intrusive to be more magnetic at this surface. A separate, surficial or near surface sheet-like body overlies the main magnetic structure. This surficial sheet produces narrow, northwesterly magnetic trends that complicate the magnetic response from the main body.

5.6 1990-1992 FIELD WORK

5.6.1 Diamond Drilling: 1990-1991

Hole # MC-1 was drilled to a final depth of 471.53 meters at grid co-ordinates L14E, 28 + OON. This hole was drilled to test a Maxi-Probe electromagnetic anomaly that coincides with an airborne magnetic anomaly. This hole is incomplete. The log of the core obtained describes the rock as being mostly shock-metamorphosed Grenville leucocratic quartzofeldspathic gneisses cross-cut by occasional small veins of meteorite-caused tachylite.

5.6.2 Property Description:

Field observations indicate that all mafic minerals in pre-existing basement rocks were transformed to magnetite within the confines of the crater delineated by the water reservoir. Post-impact ultra-

MANICOUAGAN PROPERTY

PREVIOUS WORK 18

mafic dykes were not transformed, are totally fresh, unaltered and unmetamorphosed large-grained peridotite-pyroxenites.

The suevite (fall-back breccia) is uniformly highly magnetic covers scattered regions around the crater zone and does not reflect the high magnetic signature found in the center. It is possible the oxidation of an iron-nickel meteorite could have created the highly magnetic microcrystalline dust of the suevite matrix.

A hill-sized blasted fragment of mafic basement rocks has been injected and cross-cut with tachylitic stockwerk of highly magnetic microcrystalline black material (magnetic dust?) and veins of transparent silica glass melt. L. Boivin postulates that the impact blast blew this mega-fragment into the air, limiting to a minimum the usual impact magnetization of the mafic minerals typically found in the in-situ basement rocks and in the smaller gneissic fragments caught up in the impact melt and the suevite.

The stockwerk could be either an impact tachylite of oxidized meteoritic composition or its derivative or the result of a chimney of magnetic hydrothermal fluids, such as might be found in a vent near a caldera. Company geologists confirmed the presence of significant Ni/Cu/PGE mineralization in older pre-impact mafic rocks in the crater and in very young ultra-mafic dykes cross-cutting older rocks in the crater. Geochemical assays averaged 0.79% Cu, 0.26% Ni, 2.3 gm/t Pd and 0.94 gm/t Pt. Therefore as in the case of Sudbury, the Manicouagan area was already a Cu/Ni metallogenic province prior to the meteorite impact.

5.7 1990 EXAMINATION OF THE GEOLOGICAL SETTING OF THE MAGNETIC ANOMALY - Terry Podolsky

Terry Podolsky made field visits and a geological tour of the crater and surrounding areas in company with Minéraux Manic in 1990. Subsequently, Podolsky provided conclusions to the company in a brief report which included microscopic petrography on which these assumptions are based.

The objective of this study conducted by T. Podolsky was to interpret the geological setting of the oval-shaped magnetic anomaly (Fig. 4).

Podolsky interprets the Triassic igneous rocks at Manicouagan as being part of an intrusive, the lower exposed portion of which has been modified by the assimilation of matrix and fragments of breccia (suevite). The parent melt formed in a deep-seated magma chamber where magmatism was triggered by an instantaneous decrease in lithostatic pressure as brecciated rock was ejected from the impact site to form the crater.

5.8 1991-1992 AIRBORNE AND GROUND ELECTROMAGNETIC SURVEYS

MANICOUAGAN PROPERTY

PREVIOUS WORK 19 Ground electromagnetic work performed to verify some of the Aerodat (1990) measurements and locations was initiated in 1991 and completed in 1994. An airborne survey was carried out at the same time. Delivery of the results of the airborne survey were delayed due to the total absence of detectable conductors which is uncharacteristic of the Grenville province that surrounds the impact crater.

With respect to the ground work, total field magnetometer measurements were taken in the central part of the property. A total of 340.6 km of line was cut and cut/rechained. A total of 322.1 km of measurements at 25 meter spacings were made.

Results were presented in magnetic contours drawn every 50 nanoteslas. The presence of the strong magnetic signatures in this part of the region influenced the staking position of property mining claims. It became apparent from the ground work and the subsequent digitalization and compilation of the field data that certain claim blocks are askew of the targeted magnetic highs. As a result, professional surveying of all claim block boundaries will be performed and will resolve all question of the proper location of boundaries.

The results of these surveys confirmed the rather unique geophysical nature of this structure. The results also reflect the homogeneity of a 200 million year old impact melt as well as the absence of conductors in any shocked metamorphosed Grenville basement rocks which may occur above depths of 400 vertical feet in the center of the crater. The absence of such conductors could be due to the absence of such rocks, or possibly to shock metamorphism erasure of such geophysical signatures.

5.9 1992-1993 IDENTIFICATION OF REMANENT MAGNETIZATION EFFECTS IN MAGNETIC DATA FROM THE MANICOUAGAN CRATER

Remanent magnetization can have a significant influence on the shape of the magnetic anomalies in the areas characterized by induced magnetization. This approach was applied to the magnetic anomaly in the center of the Manicouagan crater in 1992/1993 by Roest and Pilkington of the GSC.

The study consisted of proposing a method to determine the possible contribution of remanent magnetization to a particular anomaly and in measuring and re-aligning the remanent magnetization to magnetic north, thereby separating and distinguishing the total field magnetics from the magnetic field reduced to pole, the latter of which reflects more accurately the `real' magnetic intensity and dimensions of the central anomaly.

The results of this study, based only on the magnetic anomaly observations, are in agreement with constraints on the direction of remanent magnetization from rock samples. The results are significant in the debate concerning the identity and cause of the body producing the magnetic anomaly. This study suggests the anomaly was caused by the impact, in other words it was not present beforehand and did not happen after the time of impact. The magnetic anomaly, therefore cannot be a large chunk of displaced iron formation nor the topographical expression of shallow basement rocks, as had been speculated by various geologists in the past.

MANICOUAGAN PROPERTY

PREVIOUS WORK 20 5.10 1992 COMPUTER MODELLING OF MANICOUAGAN MAGNETIC DATA

Dr. N.R. Paterson of the geophysics consulting firm Paterson, Grant and Watson generated several computer models of the Manicouagan magnetic anomaly and concluded that two possible models can explain this anomaly.

1. A relatively shallow emplacement of a body of mafic composition, the center of which is not below 1,500 vertical feet, and having a magnetic susceptibility of about 4% which corresponds to a fairly well mineralized magnetic mafic intrusion.

2. A relatively deeper body, the center of which is at approximately 3,000 feet deep, with a susceptibility equivalent to 80% which, according to Mark Pilkington of the GSC, is seven times higher than that for iron formation indicating the body may be the remnant of an iron- nickel meteorite.

Dr. Paterson also concluded that the body causing the anomaly could have a minimum width of 600 meters, but more likely is 1,800 meters wide.

5.11 1992 STUDY ON MANICOUAGAN MAGNETICS

Dr. Peter Schultz, an expert on meteorite impacts in general and the Sudbury and Manicouagan structures in particular from Brown University, produced a study of the magnetic data from Manicouagan, and examined means and methods to determine the cause of the central magnetic anomaly.

Dr. Schultz hypothesizes that the differences in the crater diameter and magnetic anomaly diameter ratios between Manicouagan and other large impact sites may be due to the greater speed or smaller size and increased density of the object that hit at Manicouagan.

5.12 1994 SHADOW MAPPING OF MAGNETIC DATA

In 1994 , the Geophysics division of the GSC produced a 1:50,000 scale shadow map of the Manicouagan crater in its regional Grenville context. The relative magnetic highs and troughs are represented with respect to each other as textural features on a regional scale.

The results indicate the overall inhomogeneous pattern and variations in texture and smoothness of the magnetic signature covering the Grenvillean region to the exterior of the crater perimeters.

In contrast, the Manicouagan crater rocks, with the exception of the central anomaly which has different proportions, has a homogeneously rippled texture throughout the entire crater interior. This

MANICOUAGAN PROPERTY

PREVIOUS WORK 21 reflects a uniform magnetic background to the impact melt rocks overlying a shock metamorphosed and perhaps geophysically transformed basement.

5.13 1994 REPORT ON THE SURFICIAL GEOLOGY OF THE MANICOUAGAN STRUCTURE

The Manicouagan impact structure was mapped at 1:50,000 scale in the hopes that the nature, extent and thickness of surficial deposits would provide information useful for planning further exploration activity and subsequent property development.

The surficial deposits and the estimated thickness of the deposits over local bedrock subcrops were interpreted from stereo sets of 1:50,000 scale airphotos taken in 1976 covering a study area of 644 km-. Twelve types of surficial deposits and four depth classes were mapped and characterized. Results were presented as nine annotated airphotos and the interpretations were transferred to a 1:50,000 digitized planimetric map base.

5.14 1994 LITHOCHEMICAL STUDY OF THE MANICOUAGAN IMPACT SITE

J.M. Siriunas (1994) produced a study involving the re-evaluation of samples collected in the vicinity of the Manicouagan structure and analyzed for whole rock and trace elements. This re-examination attempted to seek lithologie evidence for the nature of the meteoric body at the Manicouagan site and/or any lithochemical anomalies that may be of economic significance.

Siriunas observed the Manicouagan melt rocks to be intermediate in the sense that they are compositionally intermediate (andesite) and that their bulk composition lies intermediate to the country rocks of the region. The composition of the melt rocks is very homogeneous. Only three samples vary chemically from the bulk composition of the melt-rock samples. Two of these samples have higher lime and magnesia contents. According to Grieve and Floran (1978), this local heterogeneity may be due to the assimilation of mafic clasts in the immediate vicinity of these samples. Siriunas notes that since the iron oxide content of these samples matches that of the bulk content, the additional lime and magnesium could have been supplied by local inclusions of limestone.

This study was inconclusive in showing that the Manicouagan melt rocks have any affinity for an extraterrestrial body, but nickel and the platinum group metals ( including iridium) were not included in the data analyzed. The geochemistry of melt rocks is not expected to reflect any association with spatially-related magmatic base metal deposits since ore-forming processes are not directly linked to the formation of an impact melt.

Siriunas suggests that continued exploration work on the property, especially in the form of diamond drilling, may intersect additional igneous lithologies on which lithochemical studies that will include nickel and the platinum group elements including iridium may supply more conclusive results.

MANICOUAGAN PROPERTY

PREVIOUS WORK 22 5.15 1996 PETROGRAPHY AND ROCK MAGNETIC PROPERTIES OF DRILL CORE FROM THE CENTRAL MAGNETIC ANOMALY

The purpose of this study, conducted by the Geological Survey of Canada (GSC), was to investigate the origin of the localized magnetic anomaly high observed in the Manicouagan impact structure by determining the lithologic, mineralogical and magnetic characteristic of a drill core taken from this magnetic anomaly. The core was analyzed for magnetic susceptibility and its anisotropy, and for natural remanent magnetization characteristics.

The following conclusions were drawn from this study. The core is composed of granulite grade basement rocks. The formation of secondary magnetite from pyroxenes and garnets resulted from the combination of the uplift of hot, high grade rocks and mechanical and thermal shock effects. The intensity of magnetization of the granulitic gneisses may have increased due to the acquisition of a new natural remanent magnetization by the shocked and heated rocks. This study determined that sulphides are present only in trace amounts. Therefore the magnetic character of the granulite gneiss sampled by the drill core and analyzed cannot be due to sulphide-type magnetic minerals such as pyrrhotite, but must be due to other magnetic minerals such as magnetite. This study also determined that alteration increases with depth which suggests that hydrothermal alteration and shock decomposition of mafic minerals extends to a greater depth than that sampled by drilling to date.

This study concludes that three-dimensional modelling of the observed magnetic field at Manicouagan and comparison with drill core measurements suggests that the high magnetizations of the causative body have not been reached by the borehole.

MANICOUAGAN PROPERTY

PREVIOUS WORK 23 6.0 DISCUSSION

Several models have been proposed to explain the characteristics of the Manicouagan structure. These models range from the Sudbury-type mineralization model to the idea that we could be dealing directly with the remains of an iron-nickel meteorite.

This last option is growing less controversial and it has been said that any object travelling at cosmic velocities and impacting over the hard rocks of the Grenville metamorphic province, would generate and release such an amount of energy that the would vaporize instantly. However, there is no proof or actual studies undertaken that would conclusively support this contention. There are scientists who claim to have found evidence of meteorites which, according to their theories, should not have survived an impact (Schultz, 1992).

Economic resources occur in approximately 25% of known terrestrial impact structures. Of these, 12% are either currently being exploited or have been exploited in the recent past. The current worth of economic materials produced from impact structures is estimated at five billion dollars per year for North America alone. Grieve and Masaitis (1994) have classified the larger economic deposits as progenetic, syngenetic and epigenetic.

Progenetic deposits include iron and uranium ore exploitable due to central uplift structures at Ternovka, Russia (375+/-25 Ma) and Carswell, Canada (115 +/- 25 Ma) as well as gold and uranium deposits in the Vredefort impact structure, especially the Witwatersrand Basin gold fields.

Syngenetic deposits include impact diamonds at several structures and Cu-Ni-PGE ores of the Sudbury Igneous Complex, interpreted as part of the impact melt system of the Sudbury Structure.

Epigenetic deposits include post-impact hydrothermal and sedimentary related deposits as well as hydrocarbon deposits such as at Ames, U.S.A.

Several facts related to the age of the impact event conspire against a typical Sudbury model. At the moment of the Sudbury event 2.1 Ga (whether this event was an explosion from exterior or interior forces is moot), the crust at the time was thin and still hot, and the forces could provoke the ascension of mantle material into near surface conditions. The later mineralization was formed mainly due to gravitational differentiation and post-event magmatism.

In the Manicouagan case, the crust was probably at least 100 km thick, and was cold and hardened, as the result of crustal differentiation and cooling over the long period of the planet's history. The impact site was located in the center of the single planetary continental craton. Therefore, it is debatable that the force of the impact was sufficient to provoke the intrusion of mantle material. However, it should have been sufficient to at least provoke the activation of post-impact magmatic events and hydrothermal recirculation of fluids in the host rocks, and probably provoke the

MANICOUAGAN PROPERTY

PREVIOUS WORK 24 remobilization of ore elements which later were concentrated on tectonically favorable environment such as faults, shear zones and breccia zones.

MANICOUAGAN PROPERTY

PREVIOUS WORK 25 7.0 CONCLUSIONS

We have collected all the available information on the Manicouagan structure and near regions. This included a meeting with Dr. Richard Grieve from the GSC, several working sessions with personnel from Great Legends Mining Inc., a complete research of the GEOREF data system and the consultation of several papers on this and related subjects.

Since there are few outcrops in the area, the main geological tools employed so far in the study of the Manicouagan impact structure are airborne geophysics and one incomplete drill hole.

The magnetic anomaly is different to any other known or similar structural feature, both in terms of size and intensity. The form of the magnetic anomaly is essentially an oval-shaped, northwest- southeast trending annulus open at the northwest end, and would conform to a geophysical response from a thick plate, slightly dish-shaped along its top surface. The upper surface of a large sharply- defined intrusive body would also create a similar response, but would require the intrusive to be much more magnetic at this surface.

It has been proven by detailed academic and scientific studies involving magnetic dating work that the anomaly did not exist prior to the meteoritic impact, nor did it happen at a later date. It was therefore brought about by the impact event.

Because of the age of these rocks we can exclude the possibility that the deposit is an iron formation, and field observation and other avenues of enquiry have eliminated for now the possibility that the anomaly is due to any topographical or metamorphic source.

In literature modelling the kinetic effects of a Manicouagan-type impact, it is postulated that the central uplift rises almost instantaneously after the impact like a piston of rock bounded by steep faults. This may explain the unique composition of the Manicouagan central uplift and may represent what underlies this immediate region at depth.

Three gravity peaks occur in the center of the crater. The two northernmost peaks are over anorthositic rocks of high elevation and the third is over a central low-lying plateau coincident with an extremely intense magnetic anomaly. The magnetic anomaly may be related to the related to the coincident gravity anomaly, which occurs over rocks different to what has generally been considered uplift rocks. Reliable in-situ rock samples in this area come from one deep drill hole which has intersected mostly leucocratic Grenville gneisses similar to those outcropping on the craton perimeter, with proportionally less massive anorthositic orthogneisses.

The models proposed to explain the causative body of the magnetic anomaly are discussed below.

MANICOUAGAN PROPERTY

CONCLUSIONS 26 1) Magnetization of a shallow ultra-mafic body located originally in the Grenville basement rocks. This model is the least plausible. Field geology has proven to contradict some of the basic premises that this model is based on.

2) A well-mineralized gabbroic or ultra-mafic shallow intrusion having a magnetic susceptibility equivalent to about a third of that of iron formation. This is a popular theory and models like that described by Orphal and Schultz (1978) for Manicouagan, suggest the possibility of a ring dyke intrusion along steeply dipping faults that would have been the result of subsidence of the central uplift peak-ring along these faults subsequent to an intrusion -related uplift into the brecciated part of the crater floor. Problems with this model are firstly, even a shallow intrusion would have a root (there is no evidence of a deep root in the magnetic modelling) and secondly, a diapir-like magma emplacement at shallow depth as a result of magmatic differentiation of continental crust would be predictably felsic.

Ultra-mafic post-impact dyke intrusions mineralized with massive Cu-Ni rich sulfides occur outside the central magnetic highs mapped for the Manicouagan anomaly. These indicate the Sudbury Intrusion Model is a possibility and a strong economic incentive for exploration.

3) Nickel-iron meteorite remnants, buried at depth between 1,500 and 12,000 vertical feet. This metallic material is seven times more magnetic than magnetite and could clearly account for the gravity, magnetic and resistivity data accumulated to date.

The meteorite model could explain a number of observed properties such as the steep gradient , sharp contacts and uniform high-intensity across large widths of the magnetic anomaly as well as the absence of a magnetic `root' and the location of the magnetic anomaly directly encircling the point of impact. This model also explains the coincidence of the gravity anomaly over the magnetic anomaly. The strongly magnetic nature of the dust matrix of the suevite and the unusually high ratios of ferric iron to ferrous iron in the whole rock lithochemistry of the impact melt rocks can also be explained.

MANICOUAGAN PROPERTY

CONCLUSIONS 27 8.0 RECOMMENDATIONS

The first step in any strategy for the study of this area is the verification of the magnetic anomaly by diamond drilling. Previous to that, some surface geophysical profile could be done in order to better position the drill holes. These holes should surpass depths of 1 to 2 km.

Samples obtained from these drill holes should be submitted to a complete suite of macro, micro and trace elements including the platinoid group. Neutron activation, ICP-MS and XRF are recommended for these studies.

Geophysical determinations of density and magnetic susceptibility should be done to study the metasomatic and hydrothermal aureoles related to the ore.

Large scale geochemical studies including stream sediments, lake sediments, soil sampling and lithochemistry should be planned for the study and evaluation of other types of mineralization in the area, including the evaluation of diamonds and gold.

Existing data should be digitalized and re-elaborated using modern modelling techniques. Any re- interpretation should take into consideration the impact theory as well as the effect of the glaciations over the area.

The Manicouagan structure represents not only a potential mineral environment, but also a significant scientific discovery, a fair amount of scientific investigation should be planned. This project should give enough research work for at least a Doctorate and a Post-Doctorate degree. There is no doubt that a better comrehension of the model of this structure will help in the future discovery of ore deposits.

A cost estimate follows on the next page.

MANICOUAGAN PROPERTY

RECOMMENDATIONS 28 INITIAL FIELD WORK:

Camp renovation, helicopter pads, logistical preparation $ 20,000 Line cutting, re-tagging, boundary surveying $ 20,000 Work permits for 1997 $ 5,000

PREPARATORY GEOPHYSICAL WORK:

Airborne data entry into the GSC database and computers to increase the resolution on previous advanced studies to refine the total observed magnetic field $ 20,000

DEEP-PENETRATING ELECTROMAGNETIC - RESISTIVITY SURVEYS:

Fine-tuning of previous work by executing not more than 15 km of surveys $ 30,000

DOWN-HOLE GEOPHYSICS:

Magnetic susceptibility measurements down-hole to provide gradient measurements for computer modelling purposes and to determine depth to summit of body,

Magnetic and/or electro-magnetic probes to bottom of hole to determine spatial orientation of magnetism $ 7,500

SURFACE GEOPHYSICAL SURVEYS:

Line cutting, 50 km at $225 per km $ 11,000 Magnetic survey along cut lines using total and gradient measurements, 75 km at $125 per km $ 9,375 Electromagnetic Max Min surveys along cut lines 50 km at $125 per km $ 6,250

PHASE I

DIAMOND DRILLING :

Drill costs for 13,500 feet (4,154 metres) in minimum 5 holes, including the completion of one hole, $100 per metre $ 415,385

MANICOUAGAN PROPERTY

RECOMMENDATIONS 29 Assay costs, overburden, fuel included. Wedges in holes extra.

Logistical costs: Helicopter and air support during campaign $ 75,000 Core shack facilities and transport costs $ 15,000 Geologists and helper salaries $ 40,000 Contingencies $ 25,240

TOTAL PHASE I $ 700,000

PHASE II

PREVIOUS WORK Compilation, Research, Reports $ 7,260 Line cutting, re-tagging $ 20,000 Work permits for 1997 $ 5,000

DIAMOND DRILLING 5 holes (near 5,000 m) @ $100/m $ 500,000 Assay cost, overburden, fuel included

DOWN-HOLE GEOPHYSICS Magnetic and/or electro-magnetic studies $ 10,000

MAPPING AND CORE LOGGING 2 senior geologists, 50 days @ $325/day $32,500 1 junior geologist, 50 days @ 100/day $ 5,000 1 prospector, 50 days @ $100/day $ 5,000

LOGISTICS Helicopter and air support during campaign $ 75,000 Core shack facilities and transport cost $ 15,000 Contingencies $ 25,240

TOTAL PHASE II $ 700,000

PHASE III 1997 EXPLORATION BUDGET

DIAMOND DRILLING 5 holes (near 5,000 m) @ $100/m $ 500,000 Assay costs, overburden, fuel included.

MANICOUAGAN PROPERTY

RECOMMENDATIONS 30 DOWN-HOLE GEOPHYSICS Magnetic and/or electro-magnetic studies $ 50,000

CORE LOGGING AND ASSAYS Assays, 5,000 samples @ $10/samples $ 50,000 Salary $ 42,500

OTHER STUDIES Geochemical studies $ 40,000 Structural analysis $ 12,260 Related expenses $ 50,000 (consultant fees, sample preparation, etc.)

LOGISTICS Helicopter and air support during campaign $ 75,000 Core shack facilities and transport cost $ 15,000 Contingencies $ 25,240

TOTAL PHASE III $ 860,000

MANICOUAGAN PROPERTY

RECOMMENDATIONS 31 9.0 REFERENCES

Berard, J., 1962. Summary geological investigation of the area bordering Manicouagan- Mouchalagane Lakes, Saguenay County. Québec Dept. Nat. Resources, P.R. No. 489, 14 pp.

Currie, K.L., 1972, Geology and petrology of the Manicouagan resurgent caldera. Geol. Surv, Can. Bull., 198, 153 pp.

Coles, R.L. and Clark, J.F.,1978, The Central Magnetic Anomaly, Manicouagan Structure, Québec. Journal of Geophysical Research, Vol.83, No. B6, pp. 2805-2809.

Dence, M.R., 1971. Impact melts. Journal of Geophysics Research, Vol. 76, pp.5552-5565.

Floran, R.J.,Grieve, R.A.F., Phinney, W.C., Warner, J.L., Simonds, C.H., Blanchard, D.P. and Deuce, M.R., 1978. Manicouagan impact melt, Québec, 1, Stratigraphy, petrology and chemistry. Journal of Geophysics Research, Vol. 83, No B6, pp. 2737-2759.

Grieve, R.A.F., and Floran, R.J., 1978. Manicouagan impact melt, Québec 2. Chemical interrelations with basement and formational processes. In Journal of Geophysical Research, Vol. 83, No. B6, pp. 2761 - 2771.

Goodfellow, W., 1992. Giant Impacts: Consequences for biological extinctions, global environments and ore formation. GEOS, Vol.21, No. 1, pp 10-11.

Grieve, R.A.F., and Floran, R.J., 1978. Manicouagan impact melt, Québec 2. Chemical interrelations with basement and formational processes. In Journal of Geophysical Research, Vol. 83, No. B6, pp. 2761 - 2771.

Grieve, R.A.F. and Masaitis, V.L., 1994. The Economic Potential of terrestrial Impact Craters. In International Geology Review, Vol.36, pp. 105-151.

GSC IPP Project # 930022, 1996. Petrograghy and rock magnetic properties of drill core from the central magnetic anomaly, Manicouagan Impact Structure. Private Report. 61 pp.

Kenwood, J. 1991. Report on Total Field Magnetics Survey of the Manicouagan Property. North- Central Québec. Private Report. 6 p.

Larochelle, A. and Currie, K.L., 1967. Paleomagnetic study of igneous rocks from the Manicouagan structure, Québec. Journal of Geophysical Research, 72, p. 4163-4169.

SOUTH-WEST MANICOUAGAN PROPERTY

BIBLIOGRAPHY 32 Murtaugh, J.G. and Currie, K.L., 1969. Preliminary study of the Manicouagan Structure. Québec Department of Natural Resources, Mines Branch, P.R.583, 9 p.

Murtaugh, J.G., 1976, Manicouagan Impact Structure; Québec Department of Natural Resources Report DPV-432,180pp.

Oronato, P.I.K., Uhlman, D.R. and Simonds, C.H., 1978. The thermal history of the Manicouagan impact melt sheet, Québec. Journal of Geophysical Research, pp. 2789-2712.

Orphal, D.L., and Shultz P.H., 1978. Manicouagan, a Terrestrial analog of Lunar Floor-Fractured craters?. In meteoritics, Vol. 13, No. 4, pp. 591 - 593.

Podolsky, T., 1990. Manic Project. Private Report. 12 p.

Robertson, W.A., 1967. Manicouagan, Québec, Paleomagnetic Results. Canadian Journal of Earth Sciences, pp. 641-649.

Roest, W.R. and Pilkington M., 1994, Manicouagan Crater. Identifying remanent magnetization effects in magnetic data. Private Report, 18 p.

Siriunas, J.M. 1994. A Lithogeochemical Study of the Manicouagan Impact Site. Private Report. 31 p.

Shultz, P.H., 1992. Private Report.

SOUTH-WEST MANICOUAGAN PROPERTY

BIBLIOGRAPHY 33 ANNEXES I

CERTIFICATE OF QUALIFICATION CERTIFICATE OF QUALIFICATION

I, Luciano Vendittelli, do hereby certify that:

1. I am a.practising consulting geologist with offices at 61 Clermont Blvd., Laval, Quebec, Canada.

2. I am a graduate of McGill University (1985), with a Bachelor of Science Degree in Geology.

3. I am a member of The Prospectors and Developers Association of Canada.

4. I have no interest direct or indirect in the properties of securities of Minéraux Manic Inc., nor do I expect to receive any such shares.

5. I have not received and I do not own any direct or indirect shares in the securities of Minéraux Manic Inc., nor do I expect to receive any such shares.

6. I have reviewed this report, pertaining documents and maps provided by Minéraux Manic Inc. I have not visited this property.

7. I am not responsible nor can I be held legally bound for any misuse or misunderstanding pertaining to this document on the Manicouagan property.

8. am a member in good standing of the A.P.G.G.Q (#1035)

Luciano Vendittelli, B.Sc., APGGQ ANNEXES II

SUPPLEMENTARY INFORMATION AND CORE LOG SUPPLEMENTARY INFORMATION AND CORE LOG

Boivin, L., 1990. Press release : Manicouagan Crater - Ni/Cu Property.

Boivin, L., 1994. Information Letter on the Manicouagan project.

Supplementary information.

Goodfellow, W., 1992. Giant Impacts: consequences for biological extinctions, global environments and ore formation. From GEOS, Vol. 21, No. 1.

Communication from Dr. N. R. Paterson on modelling of the magnetic anomaly.

Communication from Dr. Peter H. Schultz on his assessment of the Manicouagan property.

Report by Roest and Pilkington, 1993. Identifying remanent magnetization on the Manicouagan Crater.

Information on Iron Meteorites.

Core Log for DDH MC-1 drilled on the Mineraux Manic Inc. Manicouagan property in 1990-1991.

Grieve, R.A., and Masaitis, V.L., 1994. The Economic Potential of Terrestrial Impact Craters. from International Geology Review, Vol. 36. EXPLORATION MINIERE LA sARRE,Nc:

PRESS RELEASE MANICOUAGAN CRATER - NI/CU PROPERTY

December 11, 1990 Rouyn Noranda

Exploration Miniere LaSarre Inc. has mobilised a diamond drill onto the Manicouagan- nickel-copper property situated at the center of a very large meteoritic impact site north of Baie Comeau. The diamond drill program is designed to test a very strong elliptical magnetic anomaly of unknown source that is located in v.?OLt d.. srTL->•E the dead center of the crater. Drill holes will also test the very strong and wide conductors located for the most part under the magnetic anomalies at relatively shallow depths of about 500 vertical feet. These conductors can be vertical or horizontal in nature, and their causes are also unknown. The company has retained several geological models in its exploration strategy. Among these models is the well-known "Sudbury" model of mafic granitoid intrusions hosting basemetals that distinguishes the world class mining camp of Sudbury, the only other known site of meteoritic impact of similar proportions in the Western world. The company has designed other models to account for the geophysical signature witnessed on the property, and -many of them are equally encouraging for economic metal mineralization. The fact is that although much is conjectured about the nature of the phenomenon that Exploration Miniere LaSarre is about to drill, very little is known. The company will :e the first to drill a hole in the center of a meteorite impact. For this reason, both the scientific community at large and explorationists in general are focusing on this project.

/2

13, rue Gamble Est bureau 102 Ro urn-Noranda ,Quebec. J9X 3B6 Tel.: (819) 762-4364 Fax: (819! 762-9090

Exploration Results to Date

The company completed linecutting in early summer. A helicopter-borne geophysical survey was carried cut prior to field work, and geological prospecting and mapping on the property and surrounding ground was finished in August 1990. A high-resolution magnetic map was produced, and no EMH conductors were found from the air survey. This was considered to be encouraging since the company did not expect to find any conductors above the survey-penetration level of 400 vertical feet. A second and third ground geophysical survey was carried out more recently using the GEOPROBE method to penetrate the crater's horizontal layers and locate conductors. Results from this method are highly encouraging. The location of strong, wide conductors coincides with the conclusions of the field geologists who mapped and inspected the crater's geology during the summer. These conductors tend to begin at about 500 vertical feet under the surface, the same thickness estimated by the company geologists to cover the vertical basement rocks. The company predicts that mineralization of either intrusive, hydrothermal, or other origin will be found near this interface. In accordance with the Sudbury model theory, company geologists wanted to determine whether, as in the case of Sudbury, the area was already a Ni/Cu metallogenic province before the meteorite impact, and also to determine whether younger mafic intrusions of sills or dykes apt to host these metals were present. In both cases, company geologists confirmed the prescence of significant nickel/copper/platinoid mineralization in older pre-impact mafic rocks in the crater, and similar mineralization was also discovered in very young ultra-mafic dykes crosscutting older rocks in the crater. Owing to the lack of outcrops available on the LaSarre claims group, these encouraging results were-discovered elsewhere in the crater during the course of the field work, but Exploration Miniere LaSarre believes that these results are promising and vindicate their interest in this project. Some of the values obtained are as follows: Cu Ni Pd Pt Sample A: 0.67% 0.24% 2.3 gm/t 0.92 gm/t Sample B: 0.90% 0.27% 2.3 gm/t 0.96 gm/t /3 Drill Program About 10,000 feet of drilling have been planned in a total of 7 drill holes. Targets are mostly geophysical in nature, and the company expects to intersect the unexpected, i.e. new lithological units and environments.

Results from the first drill hole(s) are not expected to be available before January 1991. The drill will function until Dec. 19th, and will close down for the Christmas holidays until January 6th.

Lauri Boivin Vice President Exploration EXPLORATION MINIERE LASARRE INC. Jul, 17. 1994

Lets start the scientific discussion of the exploration merits of my project by first stating that. statistically speaking. there is probably no more favorable geological context for finding mineral deposits than meteorite impacts. And the statistics would probably be better if more p.:ople explored them for this reason alone. Of the 140 or so known terrestrial impact sites, ranging in size from small to 300 km. in diameter, 35 of them have economic deposits (more than 20'7( ), and 17 of them were in production at a a recent date. That's a better than 10g ratio in a geological environment that is not really sought after. It is really hard to figure out why not.

In North America alone, 5 - 6 billion dollars a year are produced in natural resources from impact sites. Given the relatively small number of impact sites, these geologic features have extremely high odds for finding economic potential. Of all the structures known, absolutely none of them have the outstanding magnetic drill targets of Manicouagan, and the only other sites of a size analogous to Manicouagan have very rich mineral deposits, of often world- class status i.e., Sudbury (ni-Cu-PGE), Vredefort (gold. uranium, bentonite), Kara, Russia (diamond, zinc), Popigai, Russia (diamond), Tookoonooka. Australia (oil).

1 As a function of risk, these sites are the least risky for exploration. Add to that the fact that impact geology can he rather easily and rapidly predicted, with known structural and stratigraphic and geophysical constraints, and the challenge is even less. It is a bit like arriving at Sudbury 150 years ago with our present state of knowledge.

As far as regards the scientific background. etc. the most convincing evidence to date is the accumulated magnetic studies, as well as the field observations.

To summarize the work performed:

Geomagnetic Studies:

Various academic articles and research performed since 1970's, some of which is included with the enclosed bibliography of reproduced articles, most notably:

The Central Magnetic Anomaly - Manicouagan Structure. by Coles and Clark 1978, which concludes that there are highly magnetic rocks close to the surface over an area of about 8 X 12 km.. and that the depth to the base of the body is not more than 3 km. They suggest that magnetization of a mafic body, subsequently up-lifted. to be the cause of the anomaly.

Their systematic sampling and measuring of the magnetic susceptibility of the country and melt rocks concluded that none of the samples. with the exception of two shocked ultra-mafic rocks is sufficiently magnetic to form any significant portion of the causative body of the central anomaly.

Coles and Clark discuss the cause of the magnetization of the proposed mafic or ultra-mafic body perhaps underlying the central anomaly. There is no chance that an anomaly the size of the one we see here can be caused by magnetization of anything other than an ultra-mafic body. The possiblility that the country rock in the area could have been magnetized to the necessary degree is very difficult to imagine, and science has yet to witness or observe such an occurence. They think it unlikely that thermal magnetization could have caused the magnetization as there is little evidence that the temperature below the melt rocks would be as high as the Curie temperature over a large enough volume.

They believe that a shock pressure - induced magnetization is totally adequate to produce the magnetization of mafic minerals, as such an impact can produce an order of magnitude of 20 - 3O GPa, which decreases rapidly downward and outward. They believe the steep vertical gradient of the central mag anomaly can be explained by the rapid decrease in pressure downward and outward and further delineated by rapid uplift of the affected area. If this is the case made to explain the shape and intensity of the mag anomaly then, in my opinion, two rather unlikely events may have had to occur:

i) mafic minerals at any appreciable distances from the mag anomaly would not have undergone magnetization, which is not the case, and

ii) The area directly under the mag anomaly would have to be a sharp vertically-bounded graben. representing one of several central uplifts. Although I am not aware of the existence of more than one central uplift within an impact crater, it is true that vertical structural constraints seem most reasonable, but in such a case, I would expect clear erosional boundaries to exist. with the graben either occupying a topographic low, or a high, as in the case of the anorthositic uplifted areas to the north. The topography does not delineate the mag highs in any way.

Conclusions of our field work:

a) the mafic body which would have been magnetized would have had to be the only large plug- like mafic geological unit in a rather large region. The rocks would have had to be ultra-mafic in order to produce 100% magnetite equivalent; there are no ultra-mafic units of that size in the region that I am aware of, although there are occurences of mafic gabbroic gneisses, some of them in the NE region of the crater.

h) These latter units do not demonstrate the high magnetization gradient of the central rocks, despite the fact that virtually all mafic minerals within the confines of the crater have been transformed to- magnetite, resulting in the case just mentioned._ in wide hands of almost pure magnetite. O1 course, these mafic gneisses do not have an areal extent ( at surface at least) comparable to the central area of the mag anomaly, and even if they did, buried as they would he under about several hundred feet of overlying magnetic suevite and other unknown material such as impact melt and gravel overburden, the overall magnetic signature is very considerably attenuated, in no way similar to the steep central gradient, which is itself buried.

The central rocks could not have produced a higher magnetization than other rocks of similar composition. i.e. some magnetite cannot be more magnetic than other magnetite. especially when created by the same event at the same geological time. Also, it is pretty definite that there is at least several hundred feet of overlying cover on top of the magnetic body creating the central anomaly, a circumstance which in no way appears to have attenuated in this case the steepness of the vertical magnetic gradient nor the uniformly high intensity across its entire width.

More recent magnetic work includes:

1. 1990 Aerodat survey of anomalous region, flown at 200 m. spacings. confirming and enhancing all previous measurements of the area.

2. Subsequent ground work in 1992 which verified and confirmed the airborne data.

3. Comparison work performed in 1993 by Peter Schultz of Rhode Island with our data to compare this magnetic signature with other known terrestrial impacts of like size, e.g. the Vredefort and the Sudbury structures. Schultz supports the possibilities of our models. You have a copy of one of his letters to us.

4. Work performed for us in 1992-93 by the Geological Survey of Canada, in measuring and re-aligning the remanent magnetization to magnetic north, thereby seperating and distinguishing the observed total field magnetics from the magnetic field reduced to pole, the latter which reflects more accurately the "real" magnetic intensity and dimensions of the central anomaly. A copy of a recently published article on this work is included herewith. The most important part of this work concluded beyond a shadow of a doubt that the anomaly was definitely caused by the impact, was not present beforehand, and did not happen after the impact. So, the magnetic anomaly cannot be a huge chunk of displaced iron formation, or the topographic

4 expression of shallow basement rocks, as has been speculated from time to- time by various geologists in the past.

Subsequent work has located the exact boundaries and relative intensities of this induced anomaly and chanced the location of the most highly magnetic or shallow locations of the body with respect to the observed total field. The results are no less impressive, if confidential.

I will provide you with the phone numbers of Peter Schultz and of Mark Pilkington of the GSC who performed a lot of the magnetic work. Both are experts in meteorite impacts, Mark from a geophysical viewpoint, and they can confirm the unique nature of the Manic anomly with respect to the width of high-intensity, which is extremely unusual.

Coles and Clark's conclusions also support the relative shallowness of the body ( bottom of body not deeper than 3 km) producing the magnetization. as do most recent tests performed by the University of California in lab tests ( 1991-92). These replicated the Manicouagan impact, using a metal projectile. These latter experiments determined that an iron/nickel meteorite slamming at cosmic velocity into anorthositic rocks at Manicouagan would not have penetrated more than about 3 km. deep.

5. Computer modelling of our magnetic data was performed by Dr. Paterson in Toronto with conclusions outlined in the letter you already have. His modelling papers are included herewith, and coincide pretty well with everyone else's., i.e. we are not looking at a tectonic feature, but a drill target.

As the most experienced guy in the group with relating magnetic signatures to actual geology. his strong belief that the anomaly is not an intrusion, and his statement to me that he had never in his entire life seen such an anomaly, should be given weight.

6. Our field observations indicate that:

1) all mafic minerals in pre-existing basement rocks are transformed to magnetite within the confines of the crater as delineated by the water reservoir. Post-impact ultra-mafic dykes are not transformed, of course, which is how come we assume they are post-impact occurences. These dykes are totally fresh, large-grained peridotite-pyroxenites, unmetamorphosed and unaltered. Dating these will be difficult, owing to their ultra-mafic composition and rarity of zircons.

2) the suevite (fall-back breccia) is uniformly highly magnetic, covering scattered regions around the periphery of the crater-zone, and nowhere reflects the high magnetic signature found in the centre. It is a strong possiblility that the oxidation of an iron-nickel meteorite could have created the highly magnetic microcrystalline dust of the suevite matrix.

3) A hill-sized blasted fragment of mafic basement rocks has been injected and cross-cut with tachvlytic stockwerk of highly magnetic microcrystalline black material (magnetite dust?) and veins of transparent silica glass melt. I postulate that the impact blast blew this mega-fragment into the air, thus limiting to a minimum the usual impact magnetization of the mafic minerals typically found in the in-situ basement rocks, and in the smaller gneissic fragments caught up in the impact melt and in the suevite. The (stockwerk) could be either an impact tachylite of oxidized meteoritic composition (or . a derivative thereof) or the result of a chimney of magnetic hydrothermal fluids, such as might be found in a vent near a caldera.

At any rate, one would expect a significant magnetic anomly over this feature, however. the signature , on a regional scale, is insignificant when compared with the central anomaly. More work needs -to be done here, obviously.

Other geophysical work includes:

The airborne EM Aerodat surveys, indicating no conductors above 400 vertical feet depth, make the Manicouagan crater region a naked area in a Grenvillean area where airborne INPUT anomalies are as thick as ants at a picnic,, and probably underline the difference in age, homogeneity, and nature of the 200 m.y. old Manicouagan impact rocks from the surrounding area. However, a larger area than that which we covered over the central anomaly would have to be surveyed in order to make more credible generalizations about this.

Ground EM resistivity surveys using high-penetration (2000 feet) surveys which yield very unusual profiles of extremely wide areas of low resistivity with a strong horizontal component,

6 usually under the observed in-tense magnetic fields. At first analysis, and with no petrography or other geological info to go on. cûnductivities of these anomalies could be compared to values one might expect from native nickel or iron oxide minerals, not necessarily typical copper or iron sulphides. Nobody has ever seen anomalies like these, using this instrument elsewhere in Archaean or Continental-type rocks.

Gravity surveys have been done in the past on a regional scale ( see Gravity Study of Great Impact by Sweeney. 1978 which I have included in the accompanying hiblio), and the gravity highs in the region are located dead center over the central part of the crater, which is to be expected in an impact crater, as it is modeled by uplift of more dense, less fractured or porous rocks from beneath the transient cavity. The reports concludes that the central gravity high swells gently to a high of 0 mGal over the central uplift of anorthositic gneisses.

In fact. this central high is composed of three peaks. the center lowermost one covering the magnetic anomaly to the south of the uplifted anorthositic hills, which is the point of impact of the meteorite, and where no anorthositic material outcrops. in fact where just about nothing outcrops.

The model postulated by Sweeney uses a uniform thickness of impact melt over the entire interior region of the plateau, but, in fact it is usual in an impact like Manicouagan that the impact melt is splashed out of the center to one side, and basement rocks may predominate, i.e. shocked. suevite-injected Grenvillean gneisses, which in fact is what we have intersected in our only drill hole to date, near the point of impact and under the magnetic anomaly.

The conclusions of the gravity study, which employed comet-impact and meteorite-impact models and seismic velocity data hypothesizing transient cavity depths and associated seismic horizons, would indicate that the comet model does not work, as it would require high-density surface target rocks (mafic to ultra-mafic rocks) which would be entirely too "fortuitous", to use Sweeney's term, given the relatively lower-density of the rocks in the immediate environs. ( see my comments in Conclusions starting on page one of this letter).

Conclusions would also indicate that there is no way that the transient impact cavity could have been excavated to 9 km. or greater, unless the target rocks were considerably less dense than can reasonably be expected. Instead, a cavity depth of between 2 - 3 km. is most reasonable,

7 given observed rock densities and allowing for a density horizon which could be hard to detect due to the closing of minute cracks at the transient cavity boundary. Such a boundary within the topmost few kilometers of the crust is the most reasonable conclusion of the gravity study.

This liklihood has been independantly arrived at in unconnected and more recent tests using totally different means (firing bullets in lab tests) at the University of California. The results were presented at a symposium in Sudbury in 1992.

Since, at the time of Sweeney's work, his conclusions did not really jive with accepted current theory, he examined closely the only other alternative model to explain the findings of the study, i.e. the theory that a mass of denser material, rose from below a 9 - 12 km. density horizon, to a maximum elevation of 8 km. below surface. ( which would not, in my opinion, ever produce a magnetic anomaly like we have at Manic).

But. given that the indications run contrary to a cavity depth of 9 km. or greater, and given of course the mag data, Sweeney plays around a little bit with the density parameters, significantly increasing the density contrast of the rising material by eliminating a seismic parameter, and comes to the conclusion that this kind of model could work with increased density contrast in a cavity of about 5 km. deep, and with the denser material moving upward a couple of km. having been magnetized by the impact, conforming to the possible explanation presented by Cules and Clark. 1978 - The Central Magnetic Anomalv-Manicouagan. These postulations of magnetization were briefly discussed in earlier pages.

If it is considered unlikely that highly dense material would be so conveniently located on surface target area by Sweeney, then how likely is it that such material would be present directly underneath the target at relatively shallow depth?

Also, in literature modelling the kinetic effe.cts of a Manicouagan-type impact, it is postulated that the central uplift rises almost instantaneously after the impact like a piston of rock, bounded by steep faults, from depths exceeding 10 km. This is a reasonable explanation of the fact that the central uplift has a rather unique composition for the Manicuugan area, composed almost entirely of massive moderately foliated, leucocratic anorthositic orthogneisses. This likely represents what underlies this immediate region at depth. Further work on our drill core may establish the precise depth extent of the uplift.

8 We do not have such anorthositic gneisses outcropping near the mag anomaly. nor does this area have the t-vpical topography of the Mont Babel central uplift to the north, and while there has been some controversary regarding whether or not a smaller satellite mountain of anorthosite situated to the SW of Mont Babel is an in-situ part of the central uplift, (which it is not) nobody has yet described the lower plateau region under the mag anomaly as part of the central uplift. A hump-shaped uplift with three different gravity peaks might not produce the vertical fault boundaries postulated by Cotes and Clark to explain the steep mag gradient over one of these humped peaks.

My understanding of the mechanics of the uplift would be different from what might produce an associated uplifted annular "shelf" at elevations some several hundred meters lower than the central uplift mountainous region. Of the three gravity peaks in the center of the crater, two of them ( the northernmost) are over anorthositic rocks of high elevation, and the third is over a low-lying plateau with a coincident, extremely intense magnetic anomaly.

It's about time we imagined for a moment that the mag anomaly might have something to do with the coincident gravity anomaly, which occurs over rocks with considerable differences to what has generally been considered uplift rocks. This should not be a stretch for any thinker.

The only reliable in-situ rock samples in this latter area come from one deep drill hole which has intersected mostly leucocratic Grenvillean gneisses, similar in most respects to those outcropping on the perimeters of the crater, with proportionately much much less material that can be interpreted as the massive anorthositic orthogneisses seen continually over the up-Iifted expanses of Mont Babel.

The indications that the area underlying the mag anomaly has been substantially uplifted is moot, in my opinion, at this early stage in the field work, and is not really a necessary event to postulate the probabilities of a magnetized ultra-mafic rock occuring less than 3 km. under the direct point of impact or the debatably enhanced probablities of one occurring slightly deeper, and then being uplifted.

Conclusion So here are the models we can reasonably consider to explain the causative body:

1) Magnetization of a shallow ultra-mafic body located originally in Grenvillean basement rocks. This is the hardest model to buy for many of the above reasons already discussed. But, we would have a hell of an iron deposit, maybe.

2) .A well-mineralizedgabbroic or ultra-mafic shallow intrusion having a magnetic susceptibilty equivalent to about a third of that of iron formation. ( see Paterson in business plan).

This theory is popular. and models like that described by Orphal and Schultz (1978) for Manicouagan. suggest the possibility of a ring dyke intrusion along steeply dipping faults that would have been the result of subsiding of the central uplift peak-ring along these faults subsequent to an intrusion-related uplift into the brecciated part of the crater floor.

It is a little difficult to imagine an ultra-matit or mineralized gabbroic magmatic source as shallow as 5 km., but it is undeniable that we do have ultra-mafic post-impact dyke intrusions outside of the central magnetic highs, and that they are mineralized with massive sulphide Cu-Ni rich mineralization.

These do not have 2000 nT magnetic relief, like the mag anomaly does, but the "Sudbury" intrusion model is definitely a possibility, and a strong economic incentive for exploration.

3) Nickel-iron meteorite remnants, buried at depths between 1500 vertical feet and 12,000 vertical feet. This metallic material is seven times more magnetic than magnetite and could clearly account for all the gravity,magnetic, and resistivity data accumulated to date.

Since large fragments of iron-nickel meteorites have been found and mined in South Africa, and elsewhere in areas absent of evident cratering, it may not be such a revolutionary idea to look for remnants where there is evidence of cratering and sufficient fallen object mass to have perhaps left something significant behind.

If you talk to Shoemaker, for instance, about this possibility, he will dismiss it out of hand, because as tar as he is concerned only comets can create large impact zones the size of Manicouagan. The fact nobody has ever looked for meteoritic material in big craters is not an

10 important point. All I can say is it is will be interesting what explanations will account for the total lack of H2O in the preliminary spectrometer readings coming from the impacts on Jupiter of the "comet" that is currently creating pretty humongous black eyes on that planet right now.

Peter Schultz. on the other hand. has a more open mind, and has told me he has found meteorite pieces in places where theoretically the survival of this matter was impossible.

4) The only other possible model I have come up with is the "Olympic Dam" kind of bullseve mag anomalies located over that very richly mineralized area in Australia. The anomalies are similar to those situated in your AMerican Midwest where in both cases, the mag anomalies are associated with volatile-rich hydrothermal solutions of magnetite, closely associated with rich basemetal and gold deposits, REE and other things indicative of extensive caldera activity (Olympic Dam) or crustal rifting ( Midwest ?)

Western of Australia is not talking about the cause of their very intense mag anomaly,(coincident with gravity high) because its high intensity is still not sufficiently explained, or at least, they are not talking.

Kennecott and other companies are very hot on the trail of such models. I suggest you talk to Gary Sidder of the USGS for more background. ( 303-236-5607)

Discussion

Well, I have tried to present a synopsis of some of the scientific findings and the evolution of study over the last 25 years, as objectively as possible. I reject the first model because although the technical aspects of the geophysical study is demonstrably excellent, acute ignorance of the field geology which contradicts some of the basic premises that the model is based on, as well as a rather complicated scenario relating to timing, precise faulting, the far too fortuitous prescence of partect material which is absent elsewhere, as well as a hoped-for intrusive event to explain some of the fore-going, is quite a hike for lab research geophysicists to make. More of a hike than I make with the other models.

11 The intrusive model is perfectably acceptable due to excellent field evidence of post-impact intrusives. which nonetheless occurs some distance from the central anomaly, and does not possess the mag profile of the center anomaly. Using an intrusive explanation to explain the center anomaly gives me several problems, nonetheless. Even a shallow intrusion would likely have a root, and there is no evidence of a deep root in the magnetic modelling, a diapir-like magma emplaced into a shallow locale as a result of magmatic differentiation of continental crust might he predictably felsic, tar less predictably mafic.

As for magmatic segregation or layering of the impact melt, forget it, because the impact melt is extremely homogenous compositionally-wise, and demonstrates no zoning not accountable for by a slightly more higher level of integration of anorthositic material into the melt in the northeastern half of the central impact melt plateaus. Nonetheless, we can't get around the fact that there does appear to have been some post-impact ultramafic magmatism, although no evidence of large volumes of it, and it is mineralized with the things we are looking for. Repeated hydrothermal activity along fractures under the impact from deeper sources might reasonably he expected to have caused some fluctuation somewhere in Trace element values throughout the large area sampled in lithogeochemistry studies, but there are none. So, it could he a shallow mafic, highly magnetic intrusion. But, the mag profile over the body does not cry "intrusive body" to either me or Mr. Paterson. I have a lot of field experience mapping intrusives in Archaean terrain. Of course, I've never seen a very large ring dyke complex, either, and would be quite delighted to find one here.

The Sept-Iles mafic layered intrusive located not far away to the SE, has the same dimensions as the Manicouagan crater. Both the mag and gravity data suggest a deep root, and field evidence is abundant for layering. The mag anomaly looks just like what an intrusion would normally give, and the vertical gradient clearly outlines a suspected thin, ring- dyke structure. Manic lacks anything resembling these magnetic profiles.

If you read Podalsky's report, he is certain that the impact melt is an intrusive, but I think he was basing this opinion on very outdated and subsequently well-refuted work done by the first workers in the region, and in the field I saw nothing that would confirm in my opinion that the impact melt could be anything else.

Of course, the meteorite model could explain a lot of things, such as:

12 I_ The steep gradient, sharp contacts. and uniform high-intensity across large widths of the mag anomaly.

?. The abscence of a magnetic "root".

3. The location of the mag anomaly directly encircling the point of impact and the ring structure would reflect the break-up and equidistant distribution of the big fragments.

4. The strongly magnetic nature of the dust matrix of the suevite. Geochemical analyses of the black stockwerk material might reveal the prescence of nickel. Meteoritic iron/nickel splash remnants have been found elsewhere in the world around impact sites.

5. The coincident gravity anomaly over the mag anomaly and the area which very well might not be part of the central uplift ridge. In fact, were it not for this anomaly, no one would ever have speculated that this high gravity area formed part of it.

6. The unusually high ratios of ferric iron to ferrous iron in the whole rock lithogeochemistry of the impact melt rocks which can not be explained when compared to values in the surrounding country rock. No feasible source of this almost unique discrepancy with expected results has been advanced. We must try to determine if there is any discrepency with trace elemental nickel values. Previous workers did not notice the magnetic nature of the suevite matrix, nor possibly the ubiquitous magnetization of mafic minerals in shocked basement rocks at radii of 30 km. away from the point of impact.

Iron-nickel meteorites are composed of between 20 -50 9r nickel)

I think I should decide to stop this letter. I am also sending you some info on the composition of iron meteorites, and very shortly you will receive an as yet unpublished work outlining the billions of dollars of worth already derived from the few major impacts that mankind has so far bothered to drill.

PLease call me to continue the discussion, and I hope you will decide to invest soon in this fantastic project. The management philosophy outlined in the business plan is sincere, and an

13 association with perspicacious people like yourself can add to the diminishment of controllable risks.

Sincerely yours,

Lauri Bk ivin office in Mtl. 514-845-0393 --~ «L - 4' 7 - 72 at ungodly hours, you are welcome to call 849-7692 ) Y Weekends and Mondays - 819-346-6290

14 SUPPLEMENTARY INFORMATION

Location and Access

The property is located 300 km. due north from the city of Baie Comeau on the northern shores of the St. Lawrence River, east of Quebec City. Access is by a well-maintained paved/gravelled road leading directly north from Baie Comeau to the iron mining centres northeast of Manicouagan. In about 4 hours of driving from Baie Comeau, the Relais Gabriel can be reached, a moteUrestaurant/gas bar, located on the eastern side of the Manicouagan crater at the water's edge. This motel is used as a point of departure for helicopters and bush planes westward into the heart of the crater. Our camp is about a 10 or 15 minute ride by air.

Local Infrastructure

The impressive Manic 5 hydro dam is situated just south of the crater, on the road mentioned above. Another airbase is located here. Three-phase hydro current is also readily available at the Relais Gabriel for any operations that may develop.

A railway linking Baie Comeau to Gagnonville to the north traverses the Manicouagan region not far to the east of the crater, transporting about 16 or 17 million tons of iron a year to Baie Comeau. There is a present surplus capacity of about 3-4 millions tons available on the existing freight cars, and arrangements to add new cars can be made. The railway is owned by Quebec- Cartier Mines, who operates the iron mines northeast of Manicouagan, the largest mine ore producers in Canada.

The port of Baie Comeau is the largest in Canada, shipping out about 20 million tons of resources per year, and the town is linked by highways east and west to other Quebec centres, and has regular daily business flights arriving at the city's airport. Manpower and industrial suppliers are abundant in the city.

Environmental Considerations

The property is located in the heart of the Manicouagan crater, which is accessible only by ail- from the road to the east. The few people visiting the area, are fisherman who may arrive by boat by crossing the lake, and hunters and fishermen flying into the hundreds of small and large lakes in the center of the crater during the hunting season. Visitors are rare, in fact, and our company has probably not seen more than half a dozen hunting cabins which are infrequently used.

The area has never been logged, of course, and as we are the first mining exploration team on the property, there is no environmental legacy in the form of past waste material, to deal with.

If the company should find the type of mineralization we are seeking, we would be mining a material which might be non-polluting, and not require milling or the handling of mine waste, other than sedimentation basins for water. This would be a world's first ... an environmentally- friendly base metal mine.

On the other hand, we may also discover the typical sulphide deposits of the Sudbury region, in which case it may be cheaper and more profitable to build a mill site to handle tailings etc. on the eastern edge of the crater, next to roads, hydro, and rail transport, saving investment in new enery and transport infrastructure.

There are no towns or permanent populations in the immediate area.

Native Issues

There are no native communities nearby, and the closest Indian group which one could consider as indigineous to the area, is the Montaignais nation. I have been told by the Cree Indians who are minority shareholders in one of the companies exploring the property, that the only claim the Montaignais have made is to have fishing and hunting rights in the Manicouagan area. The territories surrounding the property are a very Iong distance from any native lands of any category at all.

Also, Hydro Quebec and the province of Quebec have very important rights over the water areas and surrounding land of the Hydro reservoir, the Manicouagan lake, which surrounds the crater.

Corporate Infrastructure

Two related companies control the mineral rights of interest. One is a private company, Amadeus Resources Ltd., consisting of about 10 shareholders, all of them individual Canadians with expertise in mining exploration or production, with the exception of the Cree shareholders mentioned above, who wish to become involved in mining (even if it is very far from their own lands).

The other company is a public entity, Mineraux Manic Inc., with about one thousand shareholders, listed on the Montreal Exchange, presently not trading. There are five directors on the board, all good citizens, and all of them connected with mining, prospecting, or native relations. The private company controls the public company, and holds 549 claims in the area, while Mineraux Manic Inc. holds 216 claims in an option to purchase agreement.

Exploration Results to Date

The essential results of work performed on the target area are the following:

1. The magnetic anomaly is different to any other lmown or similar structural feature, both in terms of size and intensity.

2. It has been proven that the anomaly did not exist prior to the meteoritic impact, and was therefore caused by the meteorite's impact.

3. Experts agree that there are really only two possible scientific models to explain the magnetic signature, both of them favorable for finding an important deposit.( Any other model to explain the feature remains in the realm of scientific conjecture and imagination.)

Computer modelling and vast experience in interpreting magnetic anomalies have indicated to the experts that the body causing the anomaly is extremely large, having a width of probably 1.8 kilometers.

The body could be:

i) shallow (560 meters), and if so, an important intrusion like the Sudbury mining camp, or

ii) deeper (1000 meters), and if so, so strongly magnetic that we might be looking at the metal remanents of an iron/nickel meteorite that could be the richest known deposit of nickel or metal alloys.

4. The age of the rocks forbid the possibility that the deposit is iron formation, and studies indicate that the anomaly is not due to any topographical or metamorphic reason.

5. We have discovered two kinds of significant mineralisation; i) Sudbury-type massive sulphide vein mineralization in post-impact ultra-mafic dykes, ii) Large-scale mineralised breccia

Both types of mineralisation yield significant values of Cu-Ni-Pt values.

6. The drill hole into the prime target area is seeking a deposit richer and more important than those mentioned above, and is as yet incomplete. At 1600 feet deep, there is a good indication we have at least several hundred more feet to go. There is as yet nothing magnetic in the hole to explain the anomaly. We believe the target is deeper. QUEBEC

Ecsim k44NICCUAGAN s

45- oticNT 'fret.

/

MONTREAL ;). 1 1 f

Bassin MANICOUAGAN Basin

Profil MAGNET IQUE MAGNETIC Prof i le MANICOUAGAN

0Mm zp Energy. Mines and Énergie, Mines et qe-( 2 11•I Resources Canada Ressources Canada GCOC -,~- GIANT IMPACTS: CONSEQUENCES FOR BIOLOGICAL - EXTINCTIONS, GLOBAL ENVIRONMENTS AND ORE FORMATION

tv Wayne Goodfellow Because of the massive amount of energy form adjacent to Earth's surface and cause released, a major impact would have wildfires. Tsunamis as high as the ocean is For more than 4.5 billion years, the earth catastrophic global consequences for the deep would scour the seafloor, rework ,.has been bombarded by extraterrestrial atmosphere, hydrosphere and biosphere. sediment, form tsunami deposits and ibjects of variable size, composition and A large, heated mass of low-density air with destroy the habitat of organisms, equency of arrival. A multidisciplinary peak temperatures of 20 000 Kelvin would particularly those in shallow-water shelf effort involving geophysicists, environments. ,aieontologists, sedimentologists, analytical ^emists and geochemists has made the Earthquakes measuring SC a world leader in studying the effects 12.4 on the Richter scale or meteorite impacts on terrestrial would produce enough processes. LANDSAT image of the 'per the past 600 million years, scientists Manicouagan meteorite estimate that bodies of 5-km-diameter or impact structure, northern more hit Earth about once every 10 000 Quebec. Impact structures Tars. They estimate that the energy are characterized by circular leased from a 10-km-diameter asteroid, depressions with an uplifted outer rim and central core. :ravelling at high speed, that lands in an Melt-rock formed from the ocean 5 km deep, is more powerful than cooling of melt generated by terrestrial process, such as volcanism, impact is commonly exposed :ciation, sea level changes, earthquake in the outer rim. Image: activity or seafloor spreading. Canada Centre for Remote Sensing

de 5km est supérieure à ~ONSEQUENCES l'énergie libérée par tout processus terrestre, comme _ES IMPACTS le volcanisme, les glaciations, les variations Image LANDSAT de la structure d'impact du niveau des mers, l'activité sismique ou figAfiTESQUES météoritique de Manicouagan au Québec l'expansion des fonds marins. septentrional. Les structures d'impacts sont caractérisées par des dépressions circulaires En raison des quantités énormes d'énergie EXTINCTIONS avec une bordure extérieure et une partie libérées, un impact majeur aurait des centrale soulevées. Des roches formées après conséquences catastrophiques à l'échelle IOLOGIQUES, refroidissement de produits fondus lors de l'impact sont couramment mises à nu dans du globe pour l'atmosphère, l'hydrosohèrr l'anneau extérieur. Photo: Centre canadien de et la biosphère. Une grande masse d'air r!RANCEMENT télédétection. chauffé de faible densité, dont les températures de pointe seraient de l'ordre ANS également vanabie. Un effort de 20000 degrés Kelvin, se formerait près pluridisciplinaire de la part de de la surface de la Terre pour engendrer c!e géophysiciens. de paleontoiogues. de tempétes de feu. Dans le cas des impact< 'ENVIRONNEMENT sédimentologistes, de chimistes spécialises dans les océans, des tsunamis d'une en analyse et de géochimistes a fait de la hauteur égale à la profondeur des océans ir`IONDIAL CGC un organisme de premier plan au affouilleraient le fond marin, remanieraient niveau mondial dans l'étude des effets dei les sédiments, formeraient des dépôts de k~T FORMATION DE impacts méteorittques sur les processus tsunamis et détruiraient l'habitat des terrestres. organismes, en particulier les milieux à faible profondeur sur les plates-formes MINERAIS Les scientifiques estiment qu'au cours des continentales. dernières 60C millions d'années des corps p., `^'ayne Goodfellow d'un diamètre supérieur a Skm ont frappé la Des séismes mesurant 1 2,4 à l'échelle die Terre environ à tous les 1U 0CU ans. Ils Richter dégageraient suffisamment cari plus de 4,5 milliards d'années, la estiment également que l'énergie libérée d'énergie pour mettre en mouvement des e a été bombardée d'objets extra- lors de la chute d'un astéroïde d'un biseaux sédimentaires le long des marges e. astres de dimensions et de compositions diamètre de 1 Ckm se déplaçant à haute continentales, déclenchant ainsi ou variables dont la fréquence d'arrivée était vitesse dans un océan par une profondeur accélérant l'activité volcanique et ~ ~ DS 1992/1 energy to release sedimentary prisms along Up to 80 per cent of all life would die off as minerai deposits. Magmas formed by continental margins and initiate or a result of a meteorite impact. This is . - crustal melting during impact cause accelerate volcanism and hydrothermal because 10 to 20 per cent of ejecta dust volcanism, hydrothermal activity and the activity. Shock heating of the atmosphere and vaporized asteroid from the impact formation of magmatic, hydrothermal and would form nitrogen compounds that would remain suspended in the atmosphere sedimentary mineral deposits. The large acid rain, inhibit photosynthesis and long enough to circle the globe, block out - n ickel-copper-platinum and zinc-copper- deplete the ozone laver. sunlight, cool Earth's surface and disrupt the silver deposits at Sudbury, Ontario, are rood chain. Many researchers believe that examples of magmatic and hydrothermal Because many of the effects are similar ,o a major meteorite impact 65 million years deposits that appear to be generated by the effects of man-made pollution, scientists ago wiped out the dinosaurs and killed off meteorite impacts. The Late Devonian study meteorite impacts to attempt to about 80 per cent of all living organisms. NICK platinoid element deposit in the understand the influence of ozone The biological record is punctuated with Yukon probably formed from the depletion or acid rain on life-sustaining mass extinctions after each major disintegration_of a meteorite during impact ecosystems over periods of the usands of bombardment. Rapid evolùtion of new life and the raining of chondritic material to the. years. This helps determine the capaciry of forms filled ecological niches after each seafloor. the natural system to withstand the effects extinction. of human activity and pollution, and the Because of recent work at the CSC and rate of recovery from major environmental Meteorite impacts have also led'to the elsewhere, geologists are beginning to look damage. formation of economically important to the cosmos for answers to questions of earth evolution. The effect of impacts on Earth history is only beginning to be appreciated, initially in ~=~- ~,~~.~-~ s~_:_.,:.•~-. Schematic cross-section isolation with respect to of an impact structure the local effects of impacts, showing the excavated and more recently as they crater, outer rim and affect biological evolution distribution of ejecta ~~-•c`~`,,,F~~~~--~~.`•- and extinction, global lofted to heights up to ~==-~-+r--t>--`' --'-•- 700 km above Earth's climates and ore formation. surface. '

'pupe schématique fonte de la croûte terrestre une structure d'impact lors des impacts montrant le cratère engendrent une activité creusé, l'anneau volcanique et extérieur et 1.2 répartition hydrothermaie ainsi que la des projections souvent formation de gisements soulevées â des altitudes atteignant jusqu'à cent minéraux magmatiques. kilomètres au-dessus de hydrothermaux et la surface de la Terre. sédimentaires. Les grands gisements de nickel, cuivre hydrothern:, _. Le réchauffement météoritique. Il en est ainsi parce que et platine ainsi que de zinc, cuivre et argent nstantanc t'.,:atmosphère entraînerait la de 10 a 20%, des poussières éjectées de Sudbury (Ontario) sont des exemples de ormatior. qui et de l'astéroïde vaporisé à l'impact gisements magmatiques et hxdrothermaux produiraier: C._: pluies .'.. uie~, ininl3cr.nc nt r''stera enl en suspension dans l'atmosphère qui semblent avoir été engendrés par ces ;a pnOlosvr '^ese et e(7ui.cr.licnl l.t t Ili( ht' .assez longtemps pour faire le tour du globe impacts météoritiques. Le gisement NICK c' ozon~. et bloquer la lumière solaire, ce qui d'éléments platinoïdes du Dévonien tardif refroidirait la surface de la Terre et au Yukon s'est probablement formé suite à Puisqu'un grand nombre des effets seraient perturberait la chaîne alimentaire. Un la désintégration d'une météorite à l'impact similaires à ceux causes tsar ia pollution grand nombre de chercheurs pensent qu'un et à une pluie de matériaux chondritiques d origine anthropique, les scientifiques impact météoritique majeur a causé la sur le fond marin. e:udiant les :moacts météoritiques tenteni disparition des dinosaures et d'environ ce comprendre l'influence de l'épuisement 80 % de tous les organismes vivants il y a En raison de travaux récents menés à la de la couche d'ozone ou des pieties acides 65 millions d'années. L'enregistrement CGC et ailleurs, les géologues commencent sur les écos,,stèmes supportant la vie après biologique est ponctué d'extinctions à se tourner vers le cosmos à la recherche des intervalles de milliers d'années. Ces massives à la suite de chaque de réponses à des énigmes de l'évolution de travaux aident à déterminer l'aptitude des bombardement majeur. Une évolution la Terre. L'on commence à peine à svtèmes naturels a résister aux effets de rapide de nouvelles formes de vie vient apprécier l'effet des impacts météoritiques l'activité humaine et de la pollution ainsi combler les niches écologiques à la suite de dans l'histoire du globe, ce oui s'est que les taux de récupération après des chaque extinction. effectué initialement de manière isolée par dommages majeurs causés à l'étude d'effets locaux d'impacts puis, plus " -nvironnement. Les impacts météoritiques ont également récemment, de manière plus globale par entraîné la formation de gisements de l'examen de leur influence sur l'évolution et Jusqu'à 80% de toute vie pourrait minéraux importants sur le plan les extinctions biologiques, sur le climat de étre supprimée suite à un impact économique. Les magmas formés par la la planète et sur la formation des minerais.

CEOS 1992/1 Paterson, Grant & Watson Limited Consulting Geophysicists

FACSIMILE MESSAGE

DATE: June 18/92 # of Pages (including cover): 13

TO: Ms. Lauri Boivin Minéraux Manic

FAX NO. 1-819-764-9944 File Ref: 9257

FROM: Dr. N. R. Paterson Paterson, Grant & Watson Limited Toronto, Canada TELEFAX NO: (416) 971-7520, Phone: (416) 971-7343

Re: Manicouagan Prospect

1. I have not received the package yet with the helicopter data and drilling location.

?. Magnetic modelling of central anomaly produces the attached results.

a) The causative body could be as deep as 1000 m, with a susceptibility of 0.21 emu (roughly 80% Fe3 0z, or less)),, or as shallow as 560 m with a susceptibility of 0:01 emu Croughly 4% Fe304 or less). b) The width of the body could be a minimum of 660 m but is more likely to be close to 1800 m. Depth extent is unknown but we could determine this if we could pin down its depth.

3. The in-phase component of the EM response will be very useful in determining depth and magnetic susceptibility.

I will get back to you when I see the rest of your data.

Regards,

PATERSON, GRANT & WATSON LIMITED

Norman R. Paterson

.nn c _ - 7.-- C .tic ~c r...^ o _ - O- _. ^c 1ncnn c- •- O-• -cv B R VVN LTNIVH:I2SITY Providence. Rhnde 1x14n.d • 02912

-2ErAerMENT Of GEOLOGICAL SCIENCES 401 S63-25245. 2417, 3338

September 28, 1992

Ms. Lauri Boivin Mineraux Manic, Inc. 103-B, Rue Tremoy Quebec, J9X 1W5 CANADA

Dear Lauri:

Thanks for your letter. We were also happy to have a chance to meet you and discuss your bold project. Over the last two weeks Dave and I have been reviewing the literature, exploring modeling alternatives, and putting the snag anomalies in the context of crater-scaling relations info red from my recent work (as reported at the Sudbury conference). Our goal was to find a simplified or direct way to recognize criteria for distinguishing a shock-generated remnant field from an inductive field, to establish some sensible scaling relation for observed impact-related fields. and to establish additional tests/approaches for resolving the origin of these signatures. As you are aware, considerable work has been done to measure field strengths and to model its structure to first order. Relatively little work has been done to design field experiments that would allow more unique solutions through modern data-processing techniques. And virtually no studies have been done to place the observed fields in the context of crater-scaling relations.

Let me give you a brief summary of our assessment thus far. The bottom line is that we feel we can show that impact-generated magnetic anomalies (i.e., the amount of magnetized material) may depend simply on impactor size. Unfortunately, we cannot yet uniquely establish the role of susceptibility (i.e., unique composition). This information requires further constraints on the depth of the source region. Nevertheless, our approach could prove useful for placing Manicouagan in the framework of Sudbury and Vredefcrt, other impact sites yielding mineable ores. Without going into the details, consider the dependence of both anomaly diameter (Figure 1) and rracnetic intensity (Figure 2) as a function of the original crater diameter. Figure la clearly shows that the size of the anomaly increases essentially linearly with size with Vredefort and Sudbury anomalies being enormous. The size of the Manicouagan anomaly, however, seems small relative to crater diameter in this representation, even though the magnetic flux (magnetic field strength times anomaly area) at Manicouagan falls on the same line as Vredefort and Sudbury (Figure 2). Scaling relations (relation between crater diameter and impactor energy) indicate that crater diameter also increases approximately with impactor size. Consequently, referencing crater diameter to either impactor size or the mag anomaly should produce a straight line unless other variables enter the problem (e.g., impactor density or velocity). Figure lb shows that this seems to be the case for most of the data with the obvious exceptions of Manicouagan and Haughton. One simple and logical explanation is that the impactor forming Manicouagan (and Haughton) was either smaller or faster. In the former case, it would also have to be more dense, i.e., a smaller denser object would create a larger crater even though the size of the magnetic anomaly remains about the same (see the arrows in Figure 1). As encouraging as Figure 1 might be, the magnetic flux of the anomaly at the surface nevertheless appears to be quite normal. This inconsistency with the working hypothesis could be dismissed if the physical structure of the anomaly is different. For example, the source rock modeled as a thin plate would require increased magnetization for Manicouagan relative to other impacts in order to account for the non-anomalous magnetic flux (Figure 2). There will be a trade- off, however, between the susceptibility of region and its thickness in order to maintain the observed magnetic flux. Key inputs would be a more unique model of the anomaly (depth and thickness), better constraints on the magnetic carriers (i.e., susceptibility from the drill cores), and independent assessment of the source depth (e.g., seismic profiles).

We offer this insight to add at least credibility to the hypothesis. It can't yet be used as a proof; this may only come from your drilling program. We have actually looked into the problem in greater detail than what is described here, but much of the discussion becomes quite obtuse. You may use what's offered here to bolster your case, but I ask that it be donc sparingly since our approach is the kernel of an interesting paper.

Now in answer to your other questions. I will pass on a note to Dietz reinforcing his ideas as a worthy hypothesis. And, I would be happy to talk with some of your contacts. My approach, however, would be to emphasize that this is a viable idea, particularly in light of what we don't know and in light of your geophysical data. I think a much stronger statement could be made, however, with further constraints on the structure. In terms of matching money from scientific funding, 1 feel it is premature since government agencies are conservative and tight. A better way would be to put a student or post-doc on the problem. The only caveat would be to protect them from any possible perception by the rest of the community as doing "fringe science." This can be done with the proper goals.

In terms of publicity, this could help your cause but could hurt ours. I have learned that you can advance more quickly scientifically when you are allowed to be wrong; this isn't possible in a public forum. Nevertheless, I (we) are excited about the project and look forward to collaborating on the committee. My instincts tell me that Manicouagan will surprise us and I want to be there.

Peter H. Schultz

40

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Navigation and recovery using a Global Positioning (GPS) navigation system.

Average terrain clearance 50m Average line spacing 200m 80300. 59140. MagneticS 58850. 58700. Total Field Magnetic Intensity 58540. Contours in nT. 58490. Cesium high sensitivity 58410. magnetolTteter. . 58300. Sensor el eval I on 45m 58170. 58080. 57850. 57840. Map contours are multiples of 57740. those listed below 57670. 50 nT 57610. 250 n T 1000 of 57550. 5000 nT 57500. 57460. EM Anomalies 57410. Conductivity Thickness (nn08) 57380. 57340. e a 57290. O I - 2 57240. e 2 - 4 57170. p 4 - 8 57050. p 8 - IS 98880. • IS - 30 98820. • . 30 55650. p EM Anoma; y A, 4600 H= Iapnase anpllluae 7 ppm Cone4ctivlly InICMne,a 1-2 rtnos (sea coma).

EXPLORATIONS M I N I ERE LASARRE INC.

TOTAL FIELD MACNEYIC CONTOURS

MAN I CQUAGAN RESERVO ( R QUEBEC

SCALE I: 1 00, 000 ü 3300 5600 13200 26400 Feet

1000 2000 5000 '10000 Mt: t r es

DATE: JULY 1990

IdAERODAT LIMITED N ÏS No : 22 N/7

AA A 1.] AI,. . 7 IMPTH SECTION of App. Resistivity (ohm-metres) fron. MAk:-PROAF E. M. South North ..122 - 2 222 :2:3 2222 2227222:24222222U2222 . 222. 2aeCe2T, 2.

1

i.ca. 4 6116

• t.T, 4. TIOC

LASARRE INC SURVEY or GEOPROOE LTG OPERATOR' 0,0ERSERON iDe 00 AREA , MANICOUAGAN GRIO* NUN DATE. AA-EVETRES-TA LINE 12.0DE

Deep Electro-Maqnetic Anomaly We have:

the only geological site on Earth of same class dimensions as the ones that produced the world class mining camps of Vredefort and Sudbury, total control of the entire potential discovery site, the largest emanating undrilled magnetic anomaly in Canada, if not North America, it is a shallow drill target encompassed in a 6 X 10 km. area, i.e. this is not an academic exercise, The bullseye anomaly has the same surficial dimensions as Olympic Dam, but has a resolution of about 6,000 - 8,000 gammas above background as compared with 900 gammas for Olympic Dam, The anomaly is definitely of the same age and contemporaneous with the impact of a meteorite having about the same dimensions as the one that created the Sudbury Basin, The anomaly is therefore the result of geological processes caused immediately by the forces of the impact, i.e. hydrothermal or intrusive processes, or else by the remnants of a nickel-iron meteorite, If intrusive in nature, tests indicate we would have a very mineralized shallow body about a minimum of 600 - 1800 meters wide. If deeper, we have to be looking at a vast metallic body 7-8 times more magnetic than magnetite.

We also have:

a first drill hole with preliminary indications of a source of magnetism deeper than 1000 feet. plenty of surface mineralization resembling a description of the Francistown South African breccia - open pit , not connected with the magnetic anomalies of supreme interest. exhaustive scientific and exploration work that has narrowed the possibilities to the excellent ones described above. some of the best Canadian minefinders as shareholders the conviction we can pull drill core grading more than 12 % nickel over more than a 1000 foot widths. a business operation designed to mitigate risk and increase value for our shareholders. JOBNAME: GEO PAGE: 4 SESS: 3 OUTPUT: Mon Jan 4 07:27:31 1993 /br3/306/team8/geo/m ay93/4222 — 004

4 Roest and Pilkington reached for vertical magnetization. The correlation de- ally less than 500 nT. The exact origin of the central creases significantly when the inclination of the source magnetic anomaly is unclear although the acquisition of a magnetization is shallower. Another way of determining the strong remanent magnetization component (D = 10°E, I = difference between the analytic signal and the horizontal 40°N as compared to D = 23°W, I = 75°N for the ambient gradient function is by looking at the actual locations of their field) appears to be contemporaneous with the impact event peaks. As can be seen in Figure 3, the observed peaks of (Larochelle and Currie, 1967). Sampling of Precambrian IA(x)i and IH(x)I arc displaced with respect to one another. country rocks in the vicinity of the anomaly indicated 2 However, after reduction to the pole, both signals w susceptibilities of <10_ SI although ultramafic units sam- • a directly above the magnetization contacts. pled at greater distances revealed values up to 3.5 10 _2 SI ows the maximum distance between the peaks, âs- and remanent intensities of up to 200 A/m (Coles and Clark, of the direction of magnetization, for different 1978). On the basis of the high magnetic gradients and source layer thicknesses. The distance is minimized for subsequent modeling, Coles and Clark postulated a shallow vertical magnetization. outcropping source for the anomaly with a depth extent —3 km. MANICOUAGAN STRUCTURE ANOMALY We proceed in evaluating the significance of the remanent component of tion in the area by calculating the The study area for the application of the method is located analytic signal and the horizontal gradient of the over the central uplift of the Manicouagan impact structure, pseudogravity (_re 5c) from the data in Figure 5a. Fig- Quebec, Canada, and is centered on 51°23'N, 68°42'W. The ure Sb shows the maxima in the analytic signal, which aeromagnetic data in this area were collected at an altitude of coincide with the edges of the postulated source body. The 300 m along north/south oriented flightlines with a spacing of continuity of these maxima reflects the form of the magnetic 812 m. Figure 5a shows the magnetic field intensity data over field data and shows the highest values in the northwest a 26 x 23 km area based on data with a grid interval of corner which coincides with the shallowest part of the 200 m. The region is dominated by the intense (>2000 nT) modeled body (Coles and Clark, 1978). The horizontal anomaly that occurs in the center of the impact crater (Coles gradient maxima also approximate the body edges but dis- and Clark, 1978). The region within the boundary of the play a significant difference in amplitude between the north- impact structure, which has a diameter of 100 km, is char- ern and southern edges, which is not present in the analytic acterized by subdued magnetic relief with anomalies gener- signal. Also, in comparison with Figure 5b, the southern edge maximum in Figure 5c is displaced —1 km south of the corresponding analytic signal maxima indicated by the white a line. This difference in gradient amplitude and the displace- ment of the maxima are both indicative of a significant remanent component in the anomaly. The optimum direction of magnetization was determined by varying the reduction to the pole parameters and visually inspecting the resulting functions. The criteria used in this particular case were the balance between the northern and southern edge amplitudes and a minimal distance between the maxima of the two different functions. We found it was not possible to satisfy both criteria simultaneously for the northern edge. A com-

30 60 90 promise was found for inclination 45 degrees and declination Inclination 10 degrees (Figure 5d), which are close to the values determined from rock magnetic measurements. As indicated b by the white lines in Figure 5d, the southern boundary maxima of IH(x)I and I A(x)1 are coincident but the northern maxima of IH(x)I are still displaced northward relative to the analytic signal. This shift is most likely the result of a nonvertical northern edge of the source body, consistent with results obtained by Coles and Clark (1978). For sloping interfaces, we expect the analytic signal maxima located over the top edge and the horizontal gradient shifted down dip (Grauch and Cordell, 1987; Hansen et al., 1987). Fi re 6 shows an observed and reduced to the pole magnetic eld 30 Ei 90 120 150 180 profile across the structure (see Figure 5a for location). The Inclination difference in slope between the northern and southern edges of the source body is exemplified by the different gradients in Fic. 4. (a) Crosscorrelation coefficient (in percent) and (b) the reduced to the pole magnetic field over them. displacement (relative to z1) between the maxima in abso- Using the optimal direction of remanent magnetization, a lute values of both the analytic signal and the horizontal derivative of the pseudogravity over a single contact, for simple forward model for the Manicouagan central anomaly different values of zz/zt as a function of the direction of was constructed. The depth to source was estimated from remanent magnetization. the half-widths of the analytic signal and the horizontal JOBNAME: GEO PAGE: 5 SESS: 2 OUTPUT: Mon Jan 4 07:27:31 1993 /b r31306/te am8/geo/may9314222 —004

Identifying Remanent Magnetization

gradient function using equations (3) and (6) (see Roest et al., a starting model that can be further refined. It has the 1992b). Both methods yield similar results, but the horizon- necessary characteristics, however, and is mainly based on tal gradient is much more continuous and allows us to analysis of the magnetic field. determine the outline of the source body more accurately (compare Figures 5b and d). The depth estimates o...; d CONCLUSION from the horizontal gradient function are shown in (bottom), where the size of the circles is proportion The method presented for the analysis of magnetic anom- depth a ellr source. It clearly illustrates the shallowing of the alies is a significant help in determining a possible remarient structure towards the northwest, whereas the eastern and component. For theoretical models it is possible to minimize western edges seem to be less well resolved and probably the crosscorrelation function and derive the correct direction more deeply buried. The topography of the surface of the of magnetization. However, the magnetization declination is body was estimated by fitting a minimum curvature surface the more difficult parameter to determine, especially for high through the estimated depth values at the body edges (Figure inclination values. The application of such a fully automated 7, middle). The bottom was held fixed at 3 km, in agreement procedure to real data is complicated by variable data with the estimate of Coles and Clark (1978). The anomaly quality, especially short wavelength noise, and interference calculated over this model body is shown in the top panel of of multiple anomalies. However, visual inspection of ana- Figure 7. It shows that with this very simple model we are lytic signal and horizontal derivatives provides a good way able to match the observed anomaly quite well (compare of optimizing the remanent magnetization parameters. A with Figure 5a), further substantiating the calculated rema- simple forward model has illustrated the utility of the ap- rient magnetization direction. The model in Figure 7 is only proach.

2200 2000 1800 1600 1400 200 1200 180 1000 160 800 600 140 400 120 200 100 0 -200 80 -400 60 -600 40 -800 -1000 20 -1200 0 N 10km ~ 55 55 50 50 45 45 40 40 35 35 30 30 25 25 20 20 15 15 10 10 5 5 0 0

FIG. 5. (a) Magnetic field intensity of the central area of the Manicouagan impact structure, Quebec, Canada (51°23'N, 68°42'W). Line indicates location of profile in Figure 6, (b) Analytic signal, (c) Horizontal derivative of pseudogravity, (d) iliforizontal derivative of pseudogravity, after reduction to the pole, using a magnetization direction with inclination 45 degrees and declination 10 degrees. White lines indicate the locations of the source body edges inferred from the analytic signal. JOBNAME: GEO PAGE: 6 SESS: 3 OUTPUT: Mon Jan 4 07:27:31 1993 /br31306/team8/geo/may93/4222— 004

6 Roast and Pllkington SW NE 3000 Observed Model Anomaly i=c 2000 1 nT ~ 1800 cC t 000 1500 oE 1200 C 900 < 0 - 600 300 0 -1000 -300 0 10 20 30 600 Distance (km) -900

Fto. 6. Magnetic field profile across the Manicouagan central anomaly (see Figure 5a for location). The observed magnetic anomaly is asymmetric, with a pronounced negative to the northeast of the structure. After reduction to the pole, the northern gradient of the anomaly is less steep than the Model southern gradient, indicating a sloping contact. m 2250 ACKNOWLEDGMENTS 2000 The comments of Peter McGrath and Pierre Keating on a 1750 draft of this paper are gratefully acknowledged. We thank 1500 Walter Smith and Paul Wessel for their release of GMT 1250 graphics software used to computer generate the illustra- 1000 tions. This is Geological Survey of Canada Contribution, 750 No. 10892. 500 250 REFERENCES 0

Baranov, V., 1957, A new method for interpretation of aeromagnetic anomalies: Pseudogravimetric anomalies: Geophysics, 22, 359- 383. Bhattacharvya, B. K., 1966, A method for computing the total magnetization vector and the dimensions of a rectangular block- 10km shaped body from magnetic anomalies: Geophysics, 31, 74-96. Depth Estimates Blakely, R. J., and Simpson, R. W., 1986, Approximating edges of source bodies from magnetic or gravity anomalies: Geophysics, 51, 1494-1498. Books, K. G., 1962, Remanent magnetization as a contributor to some aeromagnetic anomalies: Geophysics, 27, 359-375. Bott, M. H. P., Smith, R. A., and Stacey, R. A., 1966, Estimation of the direction of magnetization of a body causing a magnetic anomaly using a pseudogravity transformation: Geophysics, 31, 803-811. n 2500 m Coles, R. L., and Clark, J. F., 1978, The central magnetic anomaly, C 1000 m Manicouagan structure, Quebec: J. Geophys. Res., 83, 2805- O 2808. 500 m Cordell, L., and Taylor, P. T., 1971, Investigation of magnetization and density of a North Atlantic seamount using Poisson's Theo- rem: Geophysics, 36, 919-937. Cordell, L., and Grauch, V. J. S., 1985, Mapping basement magne- tization zones from aeromagnetic data in the San Juan Basin, New Mexico, in Hinze, W. J., Ed., The utility of regional gravity and magnetic anomaly maps: Soc. Expl. Geophys., 181-197. Emilia, D. A., and Massey, R. L., 1975, Magnetization-inclination Flo. 7. Simple forward model for the Manicouagan central estimation from prism-like anomaly profiles: Geophysics, 40, anomaly; projection as in Figure S, but slightly smaller area. 443-450. Bottom: depth estimates of body edges, based on half-width Grauch, V. J, S., and Cordell, L., 1987, Limitations of determining horizontal gradient function. Only maxima in the gradient density or magnetic boundaries from the horizontal gradient of with values exceeding 20 were used (see Figure 5d). Center: gravity or pseudogravity data: Geophysics, 52, 118-121. Green, R., 1960, Remarient magnetization and the interpretation of model body with the outline determined from locations of magnetic anomalies: Geophys. Prosp., 8, 98-110. depth estimates, the topography of the surface estimated Harrison, C. G. A., 1976, Magnetization of the oceanic crust: from the depth estimates around its edges. Top: modeled Geophys. J. Roy. Astr. Soc., 47, 257-283. anomaly, showing several of the characteristics of the ob- Hall, D. H., 1959, Direction of polarization determined from mag- served anomaly (Figure 5a). netic anomalies: J. Geophys. Res., 64, 1945-1959. Hansen, R. O., Pawlowski, R. S., and Wang, X., 1987, Joint use of analytic signal and amplitude of horizontal gradient maxima for The Iron Meteorites and Pallasites

We owe the largest meteorite in captivity tee A tnan wltt> is tar better known as the lint cu reach the North Pole. In 1594, on the second of several expeditions to explore Greenland, Lieutenant— later Admiral--Robert E. Peary fell under a spell that had beguiled polar explorers since 18 I Y, when Captain John Russ of the Royal Navy found Eskimos usnig weapons and tools tippets with nickel-iron and heard stories of the "iron mountain" from whence the metal had come. Led to the site on the .:oast ut West Greenland by a hunter, Peary discovered that the "mountain" was really three huge chunks of meteoritic Eton: a 0-ion, almost wholly bill cd ivass that the Eskimos called illJlii'I tlu or the Fein,- and two smaller masses called "Lite Woman" and "the l)oe." With the determination that would take him to the Ninth Pule 15 years later, Peary set Dili to grins; the three Cape York irons hack to New York (;itv. t he lob, whi.li his l;randsusi Ilati described to vivid detail (Statford, I')8t)), entailed four summers of backbreaking work punctuated by winters of lecturing and lobbying for funds---nu small task in those pre-NASA years. But Peary persevered, and in October 1897, i'lntr,n;htlo reached New York (figure `I. I ). I gut to know the first three (tape York irons (several more pieces were found inure recently) when they were in the fover of the I Iayden Planetarium at New Yoi k (:sty's American \Inseunt of Natural His- tory. like [Oust other school chddien who visited the museum, I was awed by the immensity of "the lent" and fascinated by the ablation pits t h at dot its dark hiunte surface like giant thumbprints. Visitors can see the (.ape Yin 1: nuns to still better .advantage today in the IIIIILIt-lslnlle\ .tnd Shununt; '.t.ras I lit 11u11 \It'Icniue. .unl l'.ill.a,ua s

museum's new Arthur Ross Hall of Meteorites. As a lung-tune tan of :Ibrr{r;hint, I was disappointed to learn that .although at is the meteorite ever displayed in a museum, it Is not tie biggest ever stead.• That distinction belongs to the I-loba meteorite, which was discov- ered on .a farm in South West Attica in. 192U. I-loba measures 9 X 9 x 3'/t feet and weighs about 66 tons. Far too big to move, it still lies where it was found, though pieces of it have found their way to many museums. Iron meteorites command our .attention for several reasons, one ut which is their majestic size. Though we know of nu other chunks u metal as big as I-loba and rlhrrn'hito, one ut Western Australia's tMundrabdla meteorites weighs It) to 12 tons, and the Willamette. Oregon, iron—.also in residence at the American Museum of Natural Fliscory--weighs I.I.i tuns. In contrast, the biggest single piece of .1 stony meteorite---the Norton f aunty enstante acbondrite, which fell in Kansas in I`1•l3 -weighs barely a tun. Iron meteorites also intrigue us because our lorebears used theta tit their early efforts at metallurgy. The Eskimos' use ut the Crape York irons illustrates this, as does the scarcity of iron meteorites 'in ,much of the Middle List. As our need for some of the rarer metals that occur in such meteorites outruns terrestrial supplies, we may well writ ut the trans .al;.un or, more likely, to the asteroids from 'which they come. Rut iron meteorites and the closely related pallasues fascinate us most because they take us to the deep interiors ut their parent bodies and bear clues to processes that took place it the Earth billions. of years ago and gave our planet us metal cure and magnetic field. Irons and palllsttes also give us some of the strongest evidence we hive for the number .and sizes of meteorite parent bodies.

The \\'idtuan list ri(felt Structure

Museum curators and nleteurtte researchers often get calls Irons peo- ple who think they have round .a meteorite. Must ut these discoveries prove to be something else .ante curator built .a large t:uillcemott of such ulujeiis, which hr whnnvc file called "itcteor•wrongs"-•..-.hut es- Figure tt.! Abin Jon (- the jetty ), biggest ui the Cape York iron ruetcurtes. ccpuons ate common enough so that waist of us greet such calls with (top) Members of Peary's party struggle to take the 59-ton meteorite to the courtesy or even warmth. I low c.in we tell whether a rusty chunk of coast of Greenland. (Photograph by IA. Robert E. Peary, negative no. 32913 13 (bottom) ilbnrçbrto metal that surfaces in someone's garden is an iron meteorite-or a piece arrives at the American Mluseuru of Natural f listury. the sample and (Photographs by Orchard, negative no. -ISQttt,: of an old plow? A simple test Is to shave off .1 bit of the Department both photographs courtesy of of Library Services, ,lnterat.ua Mluscuin of Natural history.) test it for nickel, which is present in .ill meteoritic titetal. A More elegant test, which works for .all but a few Iron meteorites, as breed on 17 I I( Thundrrstuncs and tihootn,r, Sims I lie Iron Nleteorttes and I'allasnrs i

corners al cubes and in the centers of cube (aces. A natural nickel- iron alloy with this "face-centered cubic" structure is called t.unte. At lower temperatures and for low nickel contents, nickel-iron alloys adopt a second structure, in which each cube of atoms surrounds a central atom (figure 8.119. • I has .arrangement is :ailed "body- centered cubic,'' .and the corresponding natural alloy is known as kamai tae. liv slowly cooling nickel-iron .alloys in the laboratory, metallurgist have determined which polynuorph occurs at what combinations of of temperature and composition. Figure 8.4 summarizes the results this research. like the diagram for water-salt mixtures shc,nvn in the previous chapter (figure 7.1), this figure is a map in which compost• familiar coordinates of mon and teutper:ntre substitute for the [flore latitude and longitude. nutlike the salt-water diagram, this one shows only what happens to nickel-iron alloys at temperatures well below melting. To sec how the Widmannstatten pattern developed in iron Meteor- typical meteoritic composition (about ites, let us follow an alloy (il one part nickel to nine parts iron) as it cools. Figure 8.4 tells us that at has the structure high temperatures, such ;tn alloy (composition a) Figure 3.2 Polished, etched slab III the t..trbo trim meteorite (medium nct;alte• and form of taenitc. As the temperature falls, the alloy enters ,the dnte), showing bright kamacile plates separated by darker raenue to I lit Widrn,tnnst.irten structure observed in most irons. The field viewed is -IV. inches wide. (Smithsonian Institution photograph, reproduced with permis- sion.) Figure 3.3 Structures o1 cony- mum nickel-iron 'polyntorphs,' the fact that their metal has a highly characteristic texture: it consists with atoms spread ap.irt to of two nickel-iron alloys arranged na a geometrical pattern (figure show their geometrical rcla• 8.2). This pattern, discovered in 180.4 by Count Alois von Widnaann- tutinchips. Nickel :touts .are stiitten in Vienna and William Thomsen in Naples, appears when an stippled, iron .toms dear. (a) iron meteorite is sliced, polished, and etched lightly with a dilute Body-centered cubic structure solution of nitric acid in alcohol. ut nickel-poor It:uti.icire. (b) Face-centered cubic structorc fhe Wielrnannst:itten pattern both positively identifies most iron ut nickel-rich i.tenate. meteorites (a few, which contain either a great deal of nickel or very little, lack an obvious pattern) and tells us a lot .about their history. To see how this pattern funned and why it is important to our story, we have to take a short detour into another branch of science: metal- lurgy. I.ike the carbon, silica, and olivine discussed earlier in the hook, nickel-iron alloys occur in more than one crystalline form. At high temperatures and for high nickel contents, the :mums in such alloys arrange themselves in the pattern shown in Figure 8..3a: they he :at the

NIctcurites and l'all,nnrs ~ i 41 I IR • fhnndcrstones and Shooting .tit.r•, The Iron

1000 Iron-nickel alloys that contain notch less than It) percent nickel (for example, composition b Figure 8.1) evolve in much the satire way ~ C ~ as they cool, but they arrive .it a different destination. Because such 800 I alloys are iron-rich, the kamacite platcs•becom e very thick---so thick ArNI TE U F that they replad.e almost all of the tacnttc. fie structure that results K .i cubo is A consists almost wholly ut cubic crystals of kantacite. Since 600 M . that have A hexahedral (six-sided), we call nickel-pour iron meteorites C I bcxahedb•ites. I I this structure T ~ KAMACITE It is easy to see why nickel-poor iron meteorites lack the \Vidm.uut- 400 E 4- TAENITE statten structure, but why is it absent from nickel-rich meteorites as I well? To understand this, we need to look more closely at what 2000 happens when metal cools. For kamacite to Corm and grow in tacnttc; 10 20 30 40 50 nickel and iron atoms have to move through the metal, a process Nickel (Weight %) called diffusion. Atoms diffuse rapidly in high temperatures; hence losvever, :is the temperature Figure 3.4 Distribution of Lint:kuc .uni I.tr nit ui kantac to plates grow swiftly at first. I iron-nickel alloys at va nous u•ntperaIlres. based on talk, diffusion becomes Wrote and mote sluggish, causing the growth experimental studies by R F and Culdsicin of kantacite to slow down and, eventually, stop altogether. Figure 3.4 (1979). Horizontal dotted lines connect the eimipo- shows that as their hulk nickel content increases, alloys enter the sitions of the two forms of metal .0 ditfcrcnr tem- peratures. Compositions a, b, ;ind c arc appropriate to octahedrites, hexahedrires,'and araxitcs, respec- tively. Figure 9.5 Cconietrtc.tl Jistri• tuition of kaunacite plates nt a region marked "taenite + kamacite." When it does so, the taenite crystal of taenite (a). These' structure can no longer hold all of the iron present, and nickel-poor pfites parallel the faces of a regular octahedron (b). kamacite begins to form within it. At this point, the taenite has the same composition as the bulk alloy (10 percent nickel), the kamacite a more iron-rich composition (slightly inure than -1 percent nickel). As the temperature continues to fall, kamacite grows at the expense of taenite, and both minerals become more nickel-rich. for example, at 600e C, tacnttc contains about .0 percent nickel and kamacite about 6 percent. Figure 8.4 shows why iron meteorites that contain about 10 per- cent nickel consist of two nickel-iron alloys, but why du these miner- als form the distinctive crisscross pattern that we see in such meteor- ites? This structure arises because kantacite Jots nut appear at random in the taenite but grows on planes that are oriented like the faces of a regular octahedron (figure 8.5). this octahedral arrange- ment of kamacite plates gives us our naive for those iron meteorites that have an obvious WiJmannsi..itten pattern: oembeibires. dust iron meteorites are octahedriics, because most taint: nu between ti and 13 percent nickel. I'll •1 hundcrstnncs and Shooting Stars The Iron Meteorites and Pall.isitcs kamacitc t- taenitc region at lower and lower temperatures. Thus formation. Even tradirninal chemical analyses tell us little about rela- tionships among iron meteorites. for they report just a handful of the alloy b starts to form kan►acite at about 3O0° C:, but alloy c does so at 6611°. Kantacitc appears in the latter alloy at such a low temperature more abundant elements: iron, nickel, cobalt, sulfur, and small that it has very little time to grow before diffusion stops. I knee the amounts of carbon and phosphorus. kaniacite-taenitc intergrowth in a meteorite of composition c is ex- To subdivide and interpret the iron meteorites, we rely principally tremely fine-grained—so fine-grained that the meteorite appears on trace dements, which can he defined for our purposes as those structureless to the naked cyc. Our name for nickel-rich iron meteor- chemical constituents whose abundances are so low—commonly,, a, ites, ataxites, refers to this apparent lack of structure. few morns per million or billion iron atoms—that they do' nbt appear Thus all three textural varieties of iron meteorites—the abundant in traditional chemical analyses. To study these elements, chemists irradiate samples with neutrons to convert the trace elements to oitahedrites and the less common hexahedrues and .► taxites—reflect slow cooling of metal with different nickel contents. This conclusion radioisotopes; they then use the decay of these isotopes to tell which may not seem worth the long discussion that led to it, hut it is very elements are present and in what abundances. important. As we shall see later in this chapter, careful analysis of the To understand how trace elements: help us interpret the iron Widntannstiitten pattern has told us a great deal about the structures meteorites, we need to know something about the way in which a and sizes of iron meteorite parent bodies. nickel-iron liquid crystallizes. When molten nickel-iron cools, the metal that forms at each stage of crystallization contains less nickel than the associated liquid. Hence fractional crystallization produces a Classification of Iron Nlctc u nites mass of solid metal that is iron-rich at its base and progressively Students of iron meteorites divide them into eight types on the basis enriched in nickel upward (Figure 3.6). John Lovern►g showed in of texture: hexahedrites, at:txites, and six kinds of octahedrites with different thicknesses of kamacite pl.ites- This structural classification is of very long standing, and it remains useful for describing newly discovered iron meteorites. But just what do the chemical and struc- Atoxites (20%) Figure 8.6 Cross-section nt .t tural differences among iron meteorites mean? Did these meteorites differentiated nickel-iron core form in one parent body or in many bodies? Do they come from formed by fractional crystalli- centrally located cores or from small pockets of metal that arc scat- zation of a liquid composed tered through the parent bodies like the fruit in raisin bread? of II percent nickel and 39 When I examined the question of the number of chondrire parent percent iron by weight. 1 he bodies, I based my answer on three pieces of evidence: first, small but distribution of various kinds of iron meteorites in thus core sharp compositional differences among the eight or nine chondrite Medium lo Fine is based on calcul:uidns by Octahedrites groups; second, the prevalence of one-group breccias; and third and John Covering (1957). most compelling, differences of oxygen isotopic composition from (50%) one group to the next. These three arguments will be used again in Chapter 9 when i discusss the various kinds of achondrires, and shall add a fourth criterion, age, that is particularly useful for work- ing out the genealogy of those igneous meteorites. Most of the criteria that we use to work out associations of silicite- Coarse bearing meteorites are of little or no use for the irons. Though many Octahedrites iron meteorites bear the scars of shock events, few are breccias whose (20%) components might tell us which kinds of irons formed close together and which formed far apart. Fewer contain minerals that we can use Hexahedrites(I0%) to detect t•xygen isotopic differences or to establish different times of The Iran Meteorites and Pallasites a 43 I__ Thundersu►nes and Shooting Stars

1957 that fractional crystallization of a liquid with the composition gathered together in elongate fields or trends. Trend A is what one of an average iron meteorite (about I I percent nickel) can produce would expect to find in random samples of the differentiated; core solids that span the obsctvcd range of iron meteorite compositions. shown in Figure 8.6. Because iridium prefers solid metal to liquid, it is This simple process can even account for the observation that oc- most abundant in the nickel-poor cumulates that lie deep in the core; tahedrites arc -far more abundant than nickel-poor hexahedrites and its abundance decreases upward as the nickel content increases. Fields nickel-rich ataxites. B, C, and D ill Figure 8.7 show different trends of iridium aburidance How would trace elements be distributed in a differentiated metal that might result from fractional crystallization of liquids with differ- core? As we saw in the discussion of layered igneous intrusions, the ent initial compositions--for example, richer (B, C) or poorer (D):im trace elements in a magma distribute themselves among crystallizing nickel. They also show that iridium-nic,kcl diagrams and other` trace' minerals in accordance with l'auling's rules. Those ions that fit com- element diagrams can tell us a lot about crystallization history. Trend fortably in the structure of olivine, oxidized nickel for example, enter A shows the wide chemical variation that results from very efficient that mineral and follow it to the bottom of the intrusion. Other separation of crystals and liquid. In contrast, the modest variation elements that fit in feldspar, for example strontium, follow that min- evident in B indicates very slight fractional crystallization. eral. Still others, including some that are commercially important, For our present purposes, the important question is whether the find no hones in the major igneous minerals and accumulate in the iron meteorites lie on one trend in trace-clement diagrams or on many remaining liquid, from which they may eventually crystallize to form trends, that is, whether they formed from one liquid or from many ore deposits. As a result of this process of sorting trace elements liquids. Early studies of trace elements in iron meteorites were incon- according to their affinity for various minerals, samples taken from clusive on this point. When Lovering and his co-workers analyzed 38 various levels in a layered intrusion describe distinctive trends of abundance for different trace elements. Thus nickel is abundant near the bottom of such an' intrusion and decreases more or less regularly 1000 upward; strontium shows the opposite tendency. By studying trends Figure 8.7 I tvpuncetic.►l dis- for these and other trace elements, geologists can work out the crys- tributions of iridium and nickel in the core shown in' tallization history of an intrusion in considerable derail. 100 Figure 8.6 (A) and in cores The rules that govern trace clement distribution in a crystallizing that are, on average, richer magma work just as well in a crystallizing nickel-iron liquid. In fact, ([i, C) or poorer (D) in nickel. trace element trends in a metallic core arc simpler than those observed in igneous rocks, since there are fewer kinds of crystals to compete for trace elements. Thus we would expect elements that prefer solid nickel-iron to liquid to be carried down by settling crystals and con- centrated near the center of the core. Elements with the opposite preference should remain in the liquid, to be concentrated upward in the core. Figure 8.7 shows some simple relationships between iridium and nickel than might result from fractional crystallization of iron-nickel liquids with various starting compositions and histories. The first point to note in this diagram is that different scales are used for nickel (percent) and iridium (parts per million); the second is that iridium varies far more widely than nickel. Much of the value of trace de- ments lies in the fact that fractional crystallization affects them much 0.015 10 15 20 more strongly than it affects the major elements. The data points in Figure 8.7 arc not scattered across it but are NICKEL ("/o) I.! I . houulerstones .tnil '4h„„tinl', it.ns 1 he Iron Meteorites and I'allasitcrs 125 irons for gallium and germanium in l'>>i, they Iound that most of them fell mot four groups nu a diagram that compared these two elements, but that the four groups appeared to he on one line. Lover- Figure 8.8 Distribution ing and his colleagues regarded the groups as important enough to [V8 of nickel and iridium in label (I to IV), but they were inure impressed by their alignment, various chemical groups which suggested that they all came from one differentiated core. IO of iron meteorites. (Af- • The simple picture that emerged from this early trace-element study ter Scott and Wasson, 1975, reproduced by vanished when other researchers examined more elements in more permiscunr of the au- iron meteorites. Between 1967 and 1978, John Wasson and his col- thors and the American leagues at UCLA analyzed more than .SUI) irons (or gallium, ger- Geophysical Union.) manium, and iridium. Figure 3.8 summarizes their data for iridium and nickel. This figure and analogous diagrams for germanium and gallium make it clear that the iron meteorites do not fall on one trend but constitute at least 12 chemical groups. Sonic of these groups overlap in figure 8.8, but they ;ire distinct in diagrams for other 01 elements. In naming these iron groups, Wasson and his co-workers used the Itonian numerals that Lovcring had devised, adding letters to subdivide the four original groups. In cases where two groups ap- IIB peared to be related, they showed this by using two letters. Thus groups IIIA and 11111, which are similar in many respects and appear out to intergrade, appear as the field IIIAR in Figure 8.8. ti 8~ 10 12 15 20 25 Because there is no obvious way for crystal-liquid differentiation to Nickel Viol shift meteorites from one chemical group to another, Wasson and his colleagues concluded that the 12 to 16 groups defined by trace ele- Thus detailed chemical study of the iron meteorites has produced a ments sample at least a dozen parent bodies. As startling as this picture that is even more complex than the one we saw for chondrites. conclusion is, it tells only part of the story: the UCLA researchers also And as we shall see, cooling rates complicate that picture still furiher. found that about 14 percent of the iron meteorites they analyzed fell in none of the known iron groups but were scattered wildly on trace- clement diagrams. These unclassified irons may represent perhaps 50 Cores or Raisins? more groups, samples of which seldom reach the Earth. A question that trace-clement distributions do not answer is whether The groups that Wasson defined on the basis of trace-clement data the iron meteorites formed in ccntrallÿ located cores or in small pods differ in other respects as well. For example, Figure 8.8 shows that of metal that were scattered through their parent bodies. The answer some groups consist almost wholly of nickel-poor (llA and 1111) or to this question lies in the rates at which iron meteorites cooled. Irons nickel-rich (IVR) meteorites, that is, hexahedrites or ataxitcs. Others, that formed at about the same depth in a parent body—in its core, for for example IIIA and IIIR, include only octahedrites. There arc also example—should have cooled at verisimilar rates. Irons that formed mineralogical differences: most iron meteorites contain more or less at various depths, and hence were insulated by different amounts of iron sulfide (troilite) and iron-nickel phosphide (scltreibersitc), but overlying rock, should have cooled at very different rates. • members of some groups contain other minerals as well. For example, Rut how can we tell how fast an iron meteorite cooled? The answer two groups of irons contain olivine, pyroxenes, and other silicates, lies in the phenomenon that explains the very fine grained Widmann- which have yielded most of the meager data we have on the ages of stiitten structure found in nickel-rich amities: sluggish diffusion of iron meteorites. Other groups contain minerals—for example the iron and nickel at low temperatures. If diffusion were rapid at all chromium nitride carlshergite--that are known from nowhere else. temperatures, an octahcdrite would consist of homogeneous, nickel-

I 2 aundcrstooes and 'shooting Stars The Iron Meteorites and Pallasites

rich t:ac•nitc and homogeneous, nickel-poor kant.triie; we would ex- pect a series of microprobe analyses across the two minerals to show Figure 8.10 Cooling rates and a nickel distribution like Mai in Figure 8.9h. In fact, the microprobe nickel contents of various iron secs a very different pattern of nickel variation as it makes its way meteorite groups. (Frum build, across an octahcdrite (Figure 8.9c): the edge of coil► taenitc region is 1981.) nickel-rich as expected, but the nickel content drops inward, passes IV I1 through a minimum value near the center of the taenitc, and then rises toward the opposite edge. The reason for this M-shaped nickel profile teA is that during the last stages of kamacite growth, diffusion was too sluggish to carry nickel into the centers of taenitc regions. In the early 1 960s, shortly after the microprobe became available for use by mctcoriticists, two young scientists, Joseph Goldstein and John Wood, independently suggested that the shape of the M profile 1 in taenitc reflects the rate at which the metal cooled ,and can he used to estimate that rate. A very Ilat M, similar to the ideal nickel distri- 10 15 W1.%Ni bution shown in Figure 8.9b, testifies to extremely slow cooling— slow enough for diffusion to produce nearly homogeneous taenite. The sharp, deep M profile in the left-hand taenitc plate shown in bulk meteorite compositions and cooling rates, Goldstein and Wood Figure 3.9c reflects very rapid cooling, while the shallower M in the were able to translate the microprobe's observations into cooling right-hand plate of part c reflects an intermediate cooling rate. fly rates for various kinds of octahcdritcs. Their method, subsequently calculating the shapes of nickel profiles that should result for various simplified and modified by themselves and by others, has been applied to a great many iron meteorites and to tidier metal-hearing meteorites as well. Figure 8.10, which compares the nickel contents and cooling rates of several groups of iron meteorites, shows very clearly that the irons had varied thermal histories. Sonic groups, notably IIIA, IIIÇD, and ci Figure 8.9 Growth ut the Wid- mannst:iiten sin id tire• .1t various (IC, show-little variation of cooling rate, suggesting that each group rates of cooling. (a) Kant:tciie plates formed in one mass of metal—perhaps, though not necessarily, a (clear) .and aarnite (stippled) in .► hy- central core. Other groups, in particular IVr\ and [VII, have widely 'unbent-al octahrdnte, showing- the varied thermal histories, which suggest that they formed in• smaller path traversed by the nucropmbc ua masses—raisins—scattered throughout their parent bodies. I j parts Is and c. (b) Distribution of Twenty years ago metcoriticists argued almost as vehemently abi amt TI K nickel anticipated for infinitely slow 'I I b cooling or perfectly efficient diffu- whether iron meteorites formed in cores or raisins as they did about- Z II I 1 1 I I I sion. (e) Nickel distributions ex- the number and sizes of parent bodies. This issue has now faded into o ... I pected for .a high (left-hand piatc? history. As is often true in science, the answer to this "either/or" A Distance D In- .a moderate cooling rate (right- question has proved to he "both." hand plate). so

lea lliaslt es r, Although there arc only 3.5 pallasites, all but two of them finds, these olivine stony-irons arc important as a bridge between the-iron and A -- _ Q~stnnça u stony meteorites. They are also .among the handsomest of rocks, with I28 'Fhnndcrstoncs and Shooting Stars .111e Iron 1lcteoritcs and l'allasites 119'

beautifully formed, golden crystals of olivine set in gleaming metal o like topaz in a tine ring. Evidently earlier residents of the Americas 0 0 also appreciated the pallasites, for one of them—Flopewell S.I.cale S.I.cole .l 'n1.HONURITESI L iqu.d L.quid C1 Mounds—was discovered at a prehistoric burial site, and pieces of ✓ Ol.vm. others were found so far apart that we believe primitive man trans- o ported them over long distances. —3 OP ça The Thiel Mountains pallasitc, an Antarctic meteorite shown ear- lier (Figure 3.3), is typical of the class in that it consists of about two- Me lal• Melol- Sulfide Sullide IRONS I III nEll thirds ntagnesian olivine, one-third metal, a bit of Erudite and L,qu.d • L.gwd • schrcibersitc (iron-nickel phosphide), and little else. Readers with un-

usually sharp eyes may see two kinds of metal in Figure 33, for Thiel (a) (bl Mountains, like other pallasites, contains kamacite and tacnitc ar- ranged in a Widmannst:ittcn pattern. This pattern is much more obvi- Figure 8.12 Development of pallasues anti related iron (.e) Mlct.Il-sultide and ous in Figure 8.1 l, which shows a metal-rich portion of Brenham, an meteorites from .n chondnnt liquid. silicate liquids separate, with the former sinking because of unusually inhomogcneous pallasitc. its high specific gravity. (b) Fractional crystallization of the Long before meteoriticists had access to the microprobe and neu- silicate liquid produces a cumulate of settled olivine crystals tron-activation analysis, the textural similarity between the metal in just above the metal-silicate boundary. (c) As the metal- pallasitcs and that in iron meteorites Ind them to suspect that the two sulfide liquid solidifies, the utckel-rich residual liquid is kinds of meteorites arc very closely related. These modern tools have drawn upward into the olivine cumulate to produce path- confirmed that suspicion: most of the pallasites have trace-element sues. distributions and cooling rates that closely resemble those observed

for one oI the largest iron groups, IIIA13 (see Figures 8.8 and 8. 1 tl). The kw exceptions suggest that pallasites, like iron meteorites, sam- ple more than one parent body. there is still some debate about just how pallasites formed. but we have little doubt that they sample one or more boundary regions between masses ut metal and silicates--environments analogous to the Faith's core-mantle boundary. One plausible process for the for- mation of pallasites is shown iii Figure 8.12. Molten chondritic mate- rial separated into two liquids: .t metal-sulfide core and a silicate mantle (Figure 5.12x). As the nt.uule cooled, olivine crystallized early and settled out to form a cumulate just above the core-mantle bound- ary (Figure 8.12b). tiow liquid metal front the core nuived into. the olivine cumulate is less clear (Figure 8.12e). One possibility is that silicate liquid between the olivine crystals moved upward as basaltic magma, and molten metal simply replaced that liquid, just its soda'. replaces air when you suck on a straw. Another is that the mantle shrank as it cooled, squeezing molten metal out of the core and into Figure 8.11 The Brenham, Kansas, p.11lasne, showing an un- usually tidal-rich region with .t well-developed Widmann- the olivine-rich layer. st teen structure. (Smithsonian Insuouum photograph, re- We conclude, then, that most of the pallasites are samples of the produced with permission.) core-mantle boundary of the hotly that housed group Illr\Il iron I II) nunderstoncs and tihootnt:; ti1Ir, I he lion NIcteontes and l'allasires - I meteorites. The few pallasites that have unusual compositions may those of group IIAB (Figure 3.3), ire related to the enstatite chon- have formed in similar environments, but in different parent bodies. drites and .ichondritcs is that .ill three kinds of meteorites record Clearly, the IllAR parent body has sent us generous samples of its strongly reducing conditions. I lowever, some chemical differences core and lower mantle. Has it sent us bits of its upper mantle and among the iron mcteontes—for example, the high iridium contents of crust .ts well? We shall return to that question when we examine the some and the low iridium contents of others—cannot be explained in igneous achondrites in Ch ipter 9. this way. It is dear that iron meteorites formed from several different kinds of chondritic material. Before moving on to consider other differentiated meteorites, I shall ilisloi-v: Iltcntc and Variallons conclude this chapter on the irons and pallasites with a brief look at Twenty years ago it seemed possible to describe die history of iron what we know--and still do not know about two important as- meteorites quite simply: (I) a chondritic parent body started to melt; pects ut these meteorites: when they formed and what kinds of parent (2) the resulting iron-nickel-sulfur liquid trickled inward to form a bodies they came from. molten core; (3) that core solidified, with iron-iich crystals sinking to Few iron meteorites contain minerals that can br' .;.;red by conven- the center through a more nickel-rich liquid; (4) at a late stage in tional means. in fact, we have radiometric ages for only two groups, crystallization, nickel-rich liquid from the cure invaded the parent and we are not sure just what those ages mean. The most reliable ages body's lower mantle to produce those rocks that we call pallasites. now ni hand come from a small group of irons (IIE) that contain At least one group of iron meteorites--that labeled IIIAB in Fig- feldspar and can thus be dared by the very precise rubidium- ures 3.3 and 3.10—played this theme. more or less as written, but strontium method. Some of these meteorites have ages that are similar many other groups played quite elaborate variations on it. The fact to those of chondrites—about 4.55 billion years. Others have that some groups have varied cooling rates suggests that they formed younger ages, down to a minimum of 3.8 billion years for the as dispersed pockets of metal rather than central cores. One group, Kodaukanal meteorite. Taken at face value, these dates suggest that the IA13 irons, has a peculiar iridium distribution (see Figure 3.3) and the parent body of III: nteteontes started to melt very shortly after the other properties that lead us to think these meteorites were never sun funned and was still molten, at least in part, 700 million years entirely molten. Evidently these peculiar, silicate-bearing irons later. sounded just the first notes of the iron meteorite theme before they We are no longer surprised by evidence that the meteorite parent froze in a form that they have maintained for 1.5 billion years. bodies became very hot very early, but it is hard to understand how A glance at Figures 8.8 and 3.10 shows us that the various iron any of them could have staved hot for as long as 700 million years. groups differ in average composition as well as history: Some consist This is particularly puzzling in die case of irons, whose parent bodies entirely of iron-rich and some of nickel-rich meteorites, hexahcdrites were, it seems, very tiny indeed. It is unfortunate that IIE is a very .titd ataxites respectively. (dearly, these groups formed from liquids small group, for its peculiar history invites further study. • with different compositions. I low can we explain these differences? The other group of silicate-herring irons (1Ali) lacks feldspar, but As we saw in Chapter 7, differences in the availability of oxygen iodine-xenon ages of other silicates and troilite in these meteorites are could produce nickel-rich and nickel-poor liquids from one type of very high--close to those obtained for chondrites. It appears that chondritic parent material. If that material melted under oxygen-rich these meteorites, like sonic of the IlE irons, formed very early in the (oxidizing) conditions, it would produce a nickel-rich liquid and a history of the solar system. solid residue of iron-rich silicates (Figure 7.9h). If it nutted under We know nothing about the formation ages of the other iron oxygen-poor (reducing) conditions, the liquid would he much more meteorites, most of which lack silicates altogether. Fortunately, this abundant and richer in iron, and the solid residue would consist of problem has attracted the attention of nuclear chemists, who are iron-poor silicates (Figure 7.9c). exploring isotopic systems based on elements—such as palladium No doubt some of the chemical dilferences among iron meteorite and silver—that occur in meteoritic metal. If this work is successful, it groups are lue to different degrees of oxidation or reduction. Indeed, will strengthen what is now a vets weak Zink in alit knowledge of iron the reason for our suspicion that some nickel poor irons, specifically ntcteorites. I ! I

I tL drrsiotu•s and tiIruuung titats I he Irun tsictcuntcs and I'.illasrtes I

The final question that trust he asked about the vaned ohtects that for a group of irons indicate that if formed in a central core is incor- produced iron meteorites and pall.isnes is how hig they were. We saw rect, since all this really tells its is ilia( .t group id meteorites formed in Chapter 6 that we can use both pressure-sensitive minerals and about the same depth. cooling rates to determine how deeply a meteorite was buried in its parent body. If that meteorite formed at the center of the parent body, its depth of burial equals the body's radius; if not, it gives us only a Once thought to he the simplest tut meteorites, the irons have proved minimum radius for its parent body. Laboratory rxpcnments tell us on careful study to be surprisingly complex. Just how many parent than the Widmannst itten pattern cannot form at pressures greater bodies they represent is a fascinating question. Taken at face value, than about 12,000 atmospheres. Therefore, no octahedrire comes the many irons that do not fit in our present trace-clement from the center of a body with a radius greater than about 500 miles. classiticanons- about 14 percent of those studied to date—suggest This fact alone rules out parent bodies as big as the moon. Unfortu- that the Earth has sampled hits o perhaps 50 more bodies during the nately, it is impossible to use pressure arguments to narrow further last tew hundred years. Do these unclassified irons come from objects the range of permissible body sizes, because few irons contain pres- that arc just beginning to favor us with samples, or front parent sure-indicative minerals. bodies that made most of their .ontributions long ago? I lowever, most irons do contain k,umacite and tienne, and we can The ancient iron meteorites truth Antarctica may answer this ques- use the nickel distributions in these minerals to calculate cooling tion. It will he interesting to see whether some of the orphan irons will rates. These cooling rates, in turn, can be used to estimate depths of find their families among meteorites that fell thousands, even tail-, formation. When I made such calculations a few years ago (Dodd, lions, of years ago. 198 I), I concluded that those iron groups that sample cores formed in bodies with radii between 120 and 170 miles—the size of moderately large asteroids. Calculations for otheç groups yielded similar sizes, which are, of course, minima. As noted in Chapter 6, recent laboratory experiments on iron- nickel alloys have changed this picture dramatically. When we take into account the minor elements in iron meteorites, the calculated cooling rates increase sharply and the sizes we infer for parent bodies drop just as sharply. It now appears that almost all iron meteorites cooled at rates of at least 1000 C per million years. These revised cooling rates arc startling and a bit unsettling, for they imply parent bodies less than 12 miles in diameter for those iron meteorites that sample central cores. Larger htxlies are possible (or iron groups that sample "raisins," but even these meteorites appear to have formed in rather small asteroids. Thus parent bodies that we once pictured as at least as big as the moon have shrunk, in just 20 years, to objects about as big across as Manhattan Island is long. The iron meteorite parent bodies are unlikely to "shrink" further, for the new cooling-rate calculations have eliminated the principal causes of uncertainty in the old ones. In (act, I suspect that the tiny parent bodies now in view will prove to he too small: it is very hard to sec how such gravity-driven processes as fractional crystallization and core formation could take place in tiny bodies with insignificant gravitational fields. Perhaps our assumption that similar cooling rates

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The Economic Potential of Terrestrial Impact Craters

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ANn V. L. MASAI'l'IS

Karpiaskiy Geological Institute, St. Petersburg, Raisin

Abstract

Like concentrations of economic resources, terrestrial impact structures are the result of relatively rare geologic events. Economic resources occur in a number of terrestrial impact structures. After providing a context by briefly summarizing the salient points of the terrestrial impact record and the characteristics of impact craters, the relationship between impact craters and economic resources is explored. Approximately 25% of the known terrestrial impact craters are associated with some form of economic resources and— 12% currently are exploited or have been exploited in the recent past. The resources range from world-class ore deposits to relatively localized occurrences of materials. The larger economic deposits are discussed under the genetic classification of progenetic, syngenetic, and epigenetic. The progenetic deposits include the iron arid uranium ore exposed at Ternovka, Russia, and Carswell (Saskatchewan), Canada, which are exploitable because of uplift in the center of complex impact structures. Progenetic deposits also include. most of the gold and uranium deposits in the Witwatersrand Basin, South Africa, which have been protected from erosion by structures associated with the Vredefort impact structure. Syngenetic deposits include the occurrence of impact diamonds at several impact structures and the Cu-Ni•PCM ores of the Sudbury Igneous Complex, which is interpreted as part of the impact melt system of the Sudbury Structure, Ontario, Canada. Epigenetic deposits include various deposits resulting from post-impact hydrothermal and sedimentary activity. They also include hydrocarbon deposits. One of several examples is at Allies (Oklahoma), LISA, where hydrocar- bons are recovered from several Iithologies, including crystalline basement. At Ames, the impact structure not only provided the necessary structural trap but also was instrumental in localizing the source rock, which was local post•impact oil shales. The current worth of economic materials produced from impact structures worldwide is unknown, but is estimated at $5 billion per year for North America. As the known record of terrestrial impact cratering is far from complete, there remain many impact structures (and, therefore, potentially economic deposits) to be found. As such deposits are highly varied in type and impact structures are random in both geologic time and terrestrial location, it is difficult to develop au overall exploration strategy, although some generalized guidelines can be provided. Once an economic deposit has been located, however, the relatively fixed morphology and structure of impact structures of a given diameter can considerably simplify further exploration and exploitation.

Introduction impact craters and some individuals opposed vigorously the origin of specific terrestrial AS 'l'tif: I-IF:MI1T of the planetary exploration programs of the former USSR and the United structures by impact. Today, however, the States, impact craters now are recognized as a occurrence of terrestrial impact craters is common and important landform on planetary accepted more generally, largely through ,the surfaces. In parallel with this recognition, there weight of self-consistent evidence with regs d has been considerable progress in understand- to their structure, the range and distributionf ing the nature of the cratering process and its impact Iithologies (showing so-called shriek effects on planetary evolution, particularly metamorphic feat ores), and the current under- early planetary evolutimn. Prior to planetary standing o1 the physics of impact. A Instorir•rl exploration, the earth science community was discussion of the acceptance of terrestri) slow to recognize the ueeurrence of terrestrial impact structures by the general earth scient4 tor'n rim 4 "It , I:1 Irr, I. 81r1 oil II1.`i 106 It .1 F f:u1Yi7 I C/> t 1.1fAs.11'11.5 77i1W1•:S77i1,11.1111Sn le lt Ilblf~

conununit y c:ui hr found in Marvin (1000). The Quebec, I:ana 1. ). Thew types of economic current awareness of the nature:mil occurrence potential, however, can he considerable. For of terrestrial impact structures is such that new example, the hydroelectric power produced discoveries are being made by members of the from the reservoir inside the Manicouagan general geoscience conununily.just as often as structure is 1111 the order of 44)00 gigawatt hours by specialists in their study. annually. Similarly, the production of cernent The terrestrial impact record has served as a and lime products from the Rtes crater IS worth proverbial "gold mine'' aground-truth data for more than of $70 million per year. constraining the results of experiments and This paper considers examples where the calculations on the nature of the cratering pro- economic deposits are related directly to the cess (Grieve, 1991). impact is an extraordinary impact structure through structural distur- geologic process involving vast amounts of bance of the target rocks, shock heating acrd/or energy, resulting in near instantaneous rises in hydrothermal activity, and the formation of a temperature and pressure, and in the structural structural or topographic trap--that is, from redistribution of large! materials. By the same the point of view of progenetic, syngenetic, and Token, economic deposits, in general, are the epigenetic deposits. As readers may be relatively result of extraordinary geologic circumstances. wlfamilhr with the subject, we provide a con. 'Flier(' are a number of instances in which text by describing sonic of the salient charac- terrestrial impact structures have economic teristics of known terrestrial impact craters, resources-- in some rases, with considerable before discussing some examples id these types conunerrial value. Indeed, the exploration of ill deposits. The intention is not to review In the first known Ieiresuial impart crater, tier detail the considerable literature on impact ringer or Meteor (:rater, Arizona, was based in cratering, shock metamorphism, and the geo- its economic potential (Barringer, 1906). The logical and geophysical character of terrestrial principals believed that seieral million tons of impart craters. The interested reader is referred iron, nickel, and platinum group metals cmi- to such compendia and reviews as French and tained in the iron meteorite were buried beneath the crater fluor. As a commercial enter- Short (1968), Roddy et al. (1977), Masaitis et prise, however, it was a failure. For, as is now al. (1980), Mclush (1989), Grieve (1991), I'll- known, extraterrestrial bodies over ca. 107 kg kingtun and Grieve (10>12), and references impact with undiminished eusmüe' velocities. therein, for further details These velneities result in shock pressures suffi 011.111 largely. to capori7e the impacting body, and thus result in tie rllective physical removal of !taste (:hararleristie-s of III I1>11 of this material Irnimi the immediate• area Terrestrial [mime, Craters of the impart structure. In many cases, terrestrial impact craters are More recently, it has been recognized that highly degraded or noulified by other terrestrial some well-known economic deposits are located geologic processes; sorb as erosion, sedimenta- within impact structures. (if the over 1.40 tion, and, to a lesser extent, tcctonism. Never- known terrestrial impact structures (Grieve and Shoemaker, 1994). approximately 35 have sonic theless, the known sample of the terrestrial form of potentially economic deposits (Mas- impact crater population illustrates the basic aitis, 1989, 1992). The number currently (or in morphological progression will size observed the relatively recent past) producing sonic form 1111 other planets (Wood and head, 1976). 'line of economic resources is 17. in this runtrihu twit basic focus are so-called simple and com- ion, we examine the nature of some of these plex impart craters. The range of final rim •u•posïts and their relator to the formation id di;nnru•rs (/1) over who h :> (:1111. nl:1r form the impart sirm-lure. We do null consider tlu•.0 ucr its on the 11•1'1estrial plan, t -- prioiarify a siructures that lace been m- are exploited tin' Innelion of ile size of the mnp:irt event and building material- planetary gravity, with laurel luck properti,. l'1,111,1. as well ece,•eit and po•:•ubly impart yeli•nle h:nnyç;i •n•rundai y (collationr~ (1111,), .0 for hvdrnelrrlrir p wen nlloru,c (Pike , l'1}{ilI el m ,'.into, •ineplr ri .i (I•.g., l 11.li>il1- IV11 mikl. 1{11“:1:1", hlra>n>->agails I,i• n1. cur al -. , J Ill, In i a .1 kin it Ile 108 R. A. E CRIEI'!r• AN!) k l,. MA,SAITLS TF.'RRF:STRIAl. 1MI'A(.T CRATERS 109

rollout E,osionol Level etecto Meteor Croter .11

~6eotr 3t

cs

froclured a Brtcciated Target Rocks Iprvr atwcked :wcnhwwus broods - Shocked Torgel / web roe* eepmerds Rocks Low - unshodred I - IorMhonoMn Orettle n wehnut oval Irepmente '.

Fie. 3. Cross-section of a simple crater based on drillhole infrrnuatirat mainly from Brent and Mc e-ot Crater. Note presence of a lens consisting of breccias with a range of shock levels. The hulk of the breveta Is relatively unshocked, and shocked autochthonous target rocks are restricted to the base of the crater in the. center. The current erosional levels of Brent and Meteor Crater are indicated.

terrestrial simple crater is the 1.2-km-diameter Peak shock pressures occur at the point of- Barringer or Meteor Crater, Arizona, USA (Fig. impact and arc a function of impact velocity 1). Meteor Crater was formed ca. 50,000 years and the physical properties of the impacting ago (Sutton, 1985) by the impact of an iron body and the target rocks. For example the. meteorite (Buchwald, 1975), estimated to be in impact of an iron body into silicate materials at the size range of around 50 in (Schmidt, 1980). 30 km s- t results in a peak shock pressure bf ca. For an impact velocity of 25 km s-r, this corres- 1600 Gila (16 Megabars) (Ahrens and O'Keefe, ponds to the release of ca. 1016) of kinetic 1977; Melosh, 1989). ln supple craters, how energy, which is equivalent to around 60 mega- ever, the largely unshocked breccia lens mate- tons TNT. Most other terrestrial simple craters rials lie str tigraphically closer to the impact (e.g., Brent, Canada, Fig. 2) are not as well point than the shocked target rocks of the true preserved as Meteor Crater, which still retains crater floor. This apparent reversal of "shock its overturned flap of bedrock on the rim and a stratigraphy" indicates that the breccia lens large portion of its exterior ejecta blanket. materials are allochthonous in nature and has Drilling at Meteor Crater, Brent, and other led to the concept of the 'so-called "transient simple Craters Indicates that the apparent floor cavity" in the formation of simple craters. of simple craters is underlain by a breccia lens, The transient cavity is the initial cavity pro- which is approximately parabolic in cross-sec- duced on impact by the cratering flow field. The tion (Fig. 3). This lens consists of brecciated transient cavity results from the movement of target 'material and is contained in the frac- target materials by particle velocities imparted nlred, but essentially autochthonous, target to the target rocks by the shock and the;litllnw rocks of the floor and wall of the so-called true Mg release or rarefaction waves. It grows by :r Fir:. 2. Vertical aerial photograph of the simple !hem Crater, Canada. The crater is d!()1 3 Ma old and leis crater. Although the breccia kits does contain combination. of the upward and outward ext•:n•a a prevent diameter of —3 km. The original rim has been removed by erosion and the interior filled with gust• some highly shocked materials, including lion of relatively near-surface materials and tl,c impact sediments. la rnntrast to Barringer (Fig. I), Brent is slot nnwrdiarlV ...cognizable as a crater form melted Iargl•I rock, the hulk (ea. 00%) shows no downward and out Yvan! 'displacement of rehl because of terrestrial geologic processes. • diagnostic t•videnct• of shock. Shock meta- tivcly deeper materials (Fig. 4A). Various Irye) morphic effects in the autoelithonous target of detail on the otecltanirs iii simple cr:tl-i target rocks are sedimentary and up to around 4 Simple craters rocks are restricted to the floor of the true formation ran be found in Gault el al. (1968) crater (Fig. 3), where maximum shock pres- Stiiffler et al. (1975), I)cnce et al. (1077), (:col km if the target rocks arc crystallite (Dense, Simple craters consist of a bowl-shaped sures on the order of 25 Glea (25(1kilohars) are (1980), Grieve and Carvin (1984), Me•losl 1072). Above these respective diameters, ter- depression with a structurally uplifted and recorded and attenuate rapidly with depth (1989), and others. At its maximum growth, II, restrial intrtact craters have a complex form. locally overturned rim ares. The classic (Hubei-Ism' aunt ieve, 1977). transient cavil y is parabolic; in cross section an. slow to recognize the occurrence ..th epuc➢ of terreslrial imp .....V•, V• •,, . ice 01 t:rrestrrd) act structuresr by the general earth _ i enac (5)20 (3814194/43/105 EI0.(Si 103 in if the target rocks are crystalline (Dencc, 972). Above these respective diameters, ter- Simple craters consist 01 a howl-shaped i .•strial impact craters have a complex form. depression with a structurally uplifted arid sures on the order of 2: t.t'a tZ:w xnonarst ice- locally overturned rim area. The classic recorded and attenuate rapidly with depth (Robertson and Grieve, 1977). (1989), and others. At•its maximum growth, the transient cavity is parabolic in cross section and DISPLACED ZONE

44'8rENr CAVnv tTCI N

APPARENT CRATER

:humM-•G Ma.cc~i ham IC wall and ran wall num.' shocked cIa.IK and mea malenai

TRUE CRATER

FIG. 4. A. Formation of parabolic transient cavity by excavation and displacement along the flow lines of the cratering flow field (see text for more details). H. Collapse of transient cavity walls to form interior allochthonous breccia lens, resulting in the formation of the so-called apparent and true craters (see text for more details).

aches a depth roughly one-third its diameter d„ = 0.13D1 °' and 'ig. 4A). The time for the transient cavity to ach its maxium dimension at kilometer-sized d,= 0.28D'°2 raters is on the order of 10 seconds (Melosh, )89). The walls and floor of this cavity are., in where I) is final rim diameter, and d„ and d, are Etc. 5. Some examples of terrestrial complex impact craters. A. 01.1ititle aerial photograph of Sreinheini. art, lined with melted and shocked materials apparent and true depth (Figs. 3 and 4) and are ( :ernrany (3.8 kin, 15.11 I.0 Ma) with a central peak and flat floor. 13. Obligor aerial photograph of Cilssrs I in motion and driven down into the expand- defined as the maximum depth from the origi- Bluff, Australia (22 km, 142.5 ±0.5 Ma), which has been heavily modified by erosion. The prominent ring of tg cavity. The transient cavity is, however, nal ground surface to the top and bottom of the hills is 5 km in diameter and is an erosional renu,aut of a central peak wahin a much larger structure (see , Lake. (:allaita (22 km and 32 km. ighly unstable during its late-stage growth, breccia lens, respectively, and units are in kilo- text for details). C. Shuttle photograph of the twin craters in Clearvaier respectively, 290 ± 20 Ma). A central peak is submerged In the East crater and a small central peak occurs nd its walls collapse inwards. This collapse meters. The sample used to define these mor- within the ring ofislands at the West crater. D. LANDSAT image of Manicouagan, Canada (100 ken. 214 ± I nlarges the final diameter slightly cotnpared to phometric relationships includes craters in Ma). The original rim lay outside the annular lake, which is 65 km in diameter. he transient cavity diameter and partially fills both crystalline and sedimentary targets. Target he final crater with brecciated, but largely rock type. therefore, does not appear to have a these are: central peak craters (e.g., significant. For example, the pre=ent thickness inshocked, material from the transient cavity major effect on d:D relationships for simple diameter Steinheim, Germany), central peak basins with of the annular impact melt sheet at' Mani' Nall, resulting in the observed breccia lens craters. lcouagan (Fig. SD) reaches ea. 250 m over an Fig. 413). both a central peak and a surrounding ring (e.g., Complex ernlers area of 2000 km2, with an estimated original Models of the relative geometries of the tran- Red Wing Creek, USA; West Clearwater, Can- volume of approximately 1000 km3 (Phinney sient cavity and final crater have been tested Complex impact craters are characterized by ada (Fig. SC)), peak ring basins with only a ring and Simonds, 1977). In the case of Sudbury, successfully with observational data from ter- an uplifted central area, occurring as a central (e.g., Puchczli-K at unki, Russia), and possibly rest ri;ul et mule craters. I tufortnlately, only 20`7, topographic peak and/or ring (Fig. S). 'Phis is multi-ring basins with several rings (e.g., recent analyses suggest that, the so-called of the ca. 35 known simple craters have suff surrounded by an annular depression and a l'opigay, Russia). Igneous (;nmplex is part of, a-massive impact melt sheet (Taggart et al., 1985: Stiiffler et al., enl morphometric data nn crater depths, brec- normally faulted structural rim area. Although Geologic observations al complex craters cia lens volumes, and other morphometric differential erosion modifies, in some cases indicate that the central structures consist of 1989; l.akomv, 1990) with an original volume of parameters to test the model and, in some of severely, the original morphology of terrestrial shocked target rocks stratigraphically uplifted at least 8000 km3 (Grieve et al.. 1991). This rock these cases, dimensions, such as depth to the complex craters (e.g., Fig. 513), examples of the to their present position. The surrounding allochthonous material- breccia or melt base of the breccia lens. are based on the inter- morphological subgroups of complex craters, annular depression is partially filled by or a mixture—overlies the target rocks of the pretation of gravity data. The morphometry of which are more clearly observed on other plane- allochthonous target material, in the form of true crater floor, which rises through a series of the simple craters with the best available data tary bodies (Wood and Head, 1976), can be polymict breccia and/or impact melt rocks (Fig. faults to form the structurally complex rim area (Grieve et al., 1989) define the relationships: [mind in the terrestrial record. With increasing 6). The volume of impact melt rocks can be (Fig. 6).

- ...... , r..,cr,., IYoo). 1 nebe conical target rocks. to a central annular ridge 5-6 km in diameter striated fracture surfaèes'are best developed in rising roughly 200 in shove the surrounding Shock metamorphism is the progressive fine.-grained, structurally isotropic lithologies, r rr- ,err '1'1. breakdown in the stractural order of minerals such as carbonates and quartzites (Fie. 9A1- 114 R. .4. /: crttl:rt•: arw) r. f.. nr:ls.irf/s 1'Naurrs•rru: u. Id1 P1C r (ru -lulls

10.000 rock Vaporization melting

fused glossa i~ u /l'Pressure., post-shock P-T field of fyiaplectic / temperature curve for endogenic gtasse,s shock metamorphism 1000 metamorphism of granita rocks

E ~ F= / / Planar / features /

I 100 , . „1 10 100 1000 Pressure, GPa

FM. R. tug pressure-log temperature plot of fields of shock metamorphism and endogenic metamorphism. The onset pressures and associated temperatures for various shock metamorphic effects in quartz°. feldspathic crystalline rocks are indicated.

They have, however, been noted in coarser- tures have focused on quartz, because of its grained crystalline rocks, such as granites, but resistance to alteration, its relatively simple are less common and generally poorly devel- crystal structure, and its widespread occur- tic 9. Various shock metamorphic effects. A. Shatter cones in quartzite, Sudbury, Canada. It. Planar oped (Dietz, 1972). Shatter cones develop at reuce in terrestrial rocks. A recent, comprehen- deformation features in quartz., Charlevoix, Canada. C. Partial development of diaplectie feldspar glass relatively low shock pressures (Roddy and sive review of shock metamorphism in quartz is known as maskelynite (isotropicarea), St. Martin, Canada. D. Erosional remnant of impact melt rocks lining Davis, 1977), but also have been found in rocks given in Starner arid langehorst (1994). Planar the crater floor, Mistastin, Canada. Outcrop is roughly 80 m high. subjected to up to ca. 25 GPa (Milton, 1977). As deformation features, however, do occur in a result, they are generally developed in the other minerals. Specific crystallographic orien- Quaide, 1973); in infrared absorption spectra, Shock melting autochthonous rocks of the crater floor and are tations of planar deformation features are apparent as bond-length stretching (Starrier most often exposed in eroded central uplift characteristic of successive grades of shock Upon release from shtick pressures above ca. and llornemann, 1972); and in density, appar- structures of complex craters. Several studier metamorphism (Robertson et al., 1968). Stud- 50 GPa, individual minerals- begin to thermally ent as reduced density (von Engelhardt and (e.g., Manton, 1965; Milton, 1977) have indi- ies of the crystallographic orientation of planar decompose or melt (Stëffler,'1972, 1984), lead- Bertsch, 1969). cated that when the beds containing shatter ing to the production of mixed mineral melts. deformation features in quartz from terrestrial At pressures in excess of ca. 30 GPa and ca. cones at such craters arc rotated back to their Melting occurs because of the fact that not all impact structures and shock recovery experi- 40 GPa, shock-induced disorder is sufficient to pre-impact position, the majority of the apices the pressure-volume work done during shock ments have established a classification scheme, render feldspar and quartz, respectively, to a of the shatter cones point to a central location, compression is recovered on pressure release, whereby the number of sets of features, with glass (Fig. 9C). These are solid-state glasses with which occurs adiabatically. This excess work.is representing the point of impact or origin of the specific and well-characterized orientations, properties distinct from fusion glasses (Stüffler shock wave. manifest as irreversible waste heat. Above ca. 611 has been related to threshold values of shock and Hornemann, 1972). They are referred to as GPa, the waste heat 'generated on shock The most common shock metamorphic effect pressures in the range 7.5-16 GPa (Fig. 8) diaplectie or thelomorphic glasses to dis- is the occurrence of microscopic so-called (Ilürz, 1968; Robertson and Grieve, 1977; tinguish them from fusion glasses. The term decompression is sufficient to result in whole- planar deformation features in tectosilicates, StüfOer and Langehorsi, 1994). "maskelynite" is reserved for shock-induced rock melting. The resulting impact melts have most commonly in quartz (Harz, 1968). When Accompanying progressive planar-deforma- solid slate plagioclase glass (Bunch et al., compositions characteristic 9f the larger rocks fresh, the majority of these microscopic fea- tion-feature development with increasing shock 1967), which was first observed in the mete- arid occur as glass bombs in crater eject a (von tures in quartz are filled with glass (Engelhardt pressure are parallel changes in optical proper• orite Shergotty (Tschermak, 1872). In addition Engelhardt, 1990), as. glassy to crystalline and Bertsch, 1969). At older impact structures, ties, such as birefringence, refractive index, to progressive structural disorder, shock also lenses within the breccia fill, or as coherent they arc more commonly annealed and are man- optical axial angle, etc. (Starner, 1972, 1974; can generate metastable high pressure mineral annular sheets lining the; floor of complex era ifest as linear chains of microscopic inclusions Stuffier and langehorst, 1994). With the pro- polyrnorphs, e.g., stishovite and coesite from ters (Fig. OD) (Grieve et al., 1977). When (Fig. 911). Such features are called decorated gressive breakdown in ntiner l order, changes quartz (Chao, 1968) and diamond from graphite crystallized, the rucks of impact melt sheets planar deformation features (Robertson et al., occur in X •ray properties, apparent as asterism (Masaitis et al., 1972) have been documented at have igneous textures but tend to he heavily 1968). Most studies of planar deformation fea- and line broadening (Chao, 1968; llürz and terrestrial craters. charged with elastic debris towards their lower 116 !t. A. !' GRIEVE AN!) I: 1.- AfA.SAI'IZS TERRESTRIAL !A1 P1 C'P CRATERS7 Fat.S 117

Mecum/ion .Spatial distributton • Brecciation is ubiquitous at impact craters. A The active terrestrial geologic ènvironment I. 20 0- wide variety of breccias--from polymict brec- serves to remove the evidence 'of impaet crater- • eias containing a wide range of shock meta- ing through erosion, burial, or tectonism. morphosed clasts including impact melt Thus, the known record is only a small sample (known as suevites) to monomict breccias- of an originally larger population. This record is • further biased as a resplt of these terrestrial • occur as allochthonous bodies within craters processes and non-systematic search, efforts. • •• .• and as exterior ejecta, as dikes within the target • • Concentrations of known craters occur on the 120 •• rocks, and as parautochthonous bodies in the • • cratonic areas of North America, Australia, • target rocks of the crater floor. There is evi- ~ . ••• • •• Europe, and the western portion cif the former é • • dence for multi-generations, with some breccias • • USSR (Fig. 11). These are geologically stable • • • being formed during cavity excavation and oth- 80- areas with relatively low levels of erosion and M • • ers during cavity modification (Lambert, 1981; •• tectonic activity. They also are area where Muller-Mohr, 1992). Pseudotachylite also • • there have been active tlrograms to search for • occurs at large impact structures in crystalline • • • and to study impact craters. The apparent,defi- 4o targets and was first described at the Vredefort 00 20 6.0 100 lao 190 22 0 26.0 cit of known craters on othet- major cratonic DISTANCE FROM CENTER Om) structure, South Africa (Shand, 1916): blocks, such as Africa and South America .(Fig. FIG. 10. Plot of recorded shock pressure, determined from the orientation of planar deformation features Pseudotachylite and other types of breccias, 11), reflects recognition difficulties resulting in quartz, with radial distance from the center of the Charlevoix crater, Canada (estimated original diameter however, also can occur as the result of endo- from the generally lower level of knowledge of of SS km). genic geologic processes. They are, therefore, the geology and geophysics of these areas» characteristic but not diagnostic of impact. The Remotely sensed data, such as orbital imagery, and upper contacts. They may, therefore, have a the inhompgeneous distribution of meteoritic identification of shock metamorphic effects in have helped identify possible impact craters in textural resemblance to endogenic igneous material within the impact melt rocks and sam- the clasts in breccias is required before they can these areas, but ultimate confirmation of an rocks. This is not surprising, as both represent pling variations (Palme et al., 1981) or to differ- be categorically ascribed to impact. impact origin requires ground-based geologic crystallized silicate melts. In some cases, entiated, non-siderophile-enriched projectiles, observations. Approximately 30% of the known impact melt rocks were identified initially as such as basaltic achondrites (Wolf et al., 1980), terrestrial impact structures are buried. They volcanic lithologies by earlier workers. Impact which have compositions essentially indis- Distribution in Space and Time were discovered initially as geophysical anoma, lies and later drilled for, commercial or, scien- melt rocks, however, can have an unusual chem- tinguishable from terrestrial rocks. At present, there are over 140 known ter- tific purposes. istry as compared to endogenic volcanic rocks. restrial impact craters (Grieve and Shoemaker, Shock attenuation The incompleteness of ,the known record is Their chemistry is governed by melting of some 1994). This number does not include the exemplified by considering the marine record mix of target rocks, as opposed to partial melt- Because of attenuation, diagnostic shock numerous small (<10 in) impact pits that of impact. Only one confirmed impact crater is ing and/or fractional crystallization relation- metamorphic effects in autochthonous rocks appear to be associated with recent meteor currently known in the world's oceans. This is ships in the case of endogenic igneous rocks. are riot observed in the rim area of simple showers. New discoveries are made at the rate the 45-km-diameter, 50-m.y. Montagnais struc- Isotopic analyses also indicate that such param- craters and are limited in complex craters to of 3-5 impact craters per year, with major ture on the continental shelf off Nova Scotia, eters as 87Sr/86Sr and i43Nd/t44Nd ratios reflect radial distances of < 0.3 the final diameter increases in the number of discoveries having Canada (Fig. 11) (Jansa et al., 1989). Mon- the pre-existing target rocks, while isochron (Robertson and Grieve, 1977). That is, they taken place in the last 20 years in Australia and tagnais was discovered and drilled in the course dating methods indicate much younger crystal- occur only in the central area of the trite crater the former USSR. There also are a number of of hydrocarbon exploration. Anotherstructlire, lization ages related to the impact event (e.g., floor, i.e., at the base of the breccia lens in probable impact craters, which have several of Mjdinir (39 km in diameter), has been defined. Jahn et al., 1978; Faggart et al., 1985). simple craters and in the central uplifted struc- the characteristics of known impact craters but in seismic data in the Barents Sea Enrichments above target rock levels in trace ture and immediately surrounding area at com- lack definitive evidence such as the occurrence (Cudlaugsson, 1993). It has not. been drilled to siderophile and platinum group elements, and plex craters. They diminish in intensity with of shock metamorphic features. As more infor- date and has not been confirmed .as an impact sometimes Cr, have been identified in some radial distance from the center of the crater mation becomes available on these structures, it structure. No impact craters are known in the impact melt rocks (Palme, 1982). These are due (Fig. 10) and with depth. They also occur, of is likely that some will acquire a confirmed trite oceanic crust of the ocean basins. This to an admixture of up to a few percent of course,, in allm -htlionous -lithologies, such as status. The impart origin of all these st letlires, reflects Zack of knowledge Of the detailed meteoritic projectile material. In same cases, breccias and melt rocks. Not all terrestrial cra- however, is not universally accepted and there topography and geology of the world's ocean the relative abundances of the various trace ters have preserved the full spectrum of shock are still a few geologists who have difficulty basins. elements in impact melt rocks have constrained metamorphic effects. Deeply eroded craters accepting the occurrence of terrestrial impact Temporal Ih.sir i bll t iorl the projectile responsible to the level of mete- have a reduced spectrum of effects, limited in craters, preferring to ascribe their origin to orite class. In other cases, no geochernical autochthonous rocks to the lower grades of various tectonic-cryptovolcanic processes There is a clear bias its the ages of terrestrial anomaly has been identified. This may be due to shock metamorphism. (Nicolaysen and Ferguson, 1990). impact craters. over 50% of rite known craters 118 R. A. F. GRIEVE AND f.' L. AfASA1TL5 TERRFSTRIAI. IMPACT CRATERS 119

TABLE 1. Genetic Groups of Economic Deposits

Genetic group Principal mode of origin Types of known deposits

Progenreic Itrecciation Earth silica Structural displacement Iron. uranium, gold

Syngenctic Phase transitions Impact diamond Crustal melting Co, Ni. PCMt, glass

Epigenetic Hydrothermal activity Lead, zinc, uranium, pyrite, zeolite. apte Sedimentation Placer diamond and tektites Chemical and biochemical sedimentation Zeolite, bentonite, evaporites, oil ahato, diatomite, lignite, amber, calcium phosphate Underground flow Oil, natural gas, fresh and mineralized water

'PGM = platinum-group metals.

(Zeylick and Seytmuratova, 1974; Saul, 1978) contains remnants of allôchthonoi►s breccia and that massive basin-forming impact events, with patelles and lenses a suevite. The present which must have occurred early in earth his- diameter of the structure is roughly 1.0-:11 km tory, affected subsequent magmatic and hydro- Re. 11. Spatial distribution of currently known terrestrial impact craters (see text for details regarding its (Val'ter, 1988) and its original diameter may thermal activity to produce distinct significance). have been 1 5- 18 km. metallogenic provinces in areas of the earth's The iron ores at Ternovka have been were formed more recently than 200 Ma. As in any pateoenvironment, the resultant eco- crust (Zeylick and Seytmuratova, 1974; Saul, exploited for over 50 years. Originally, thel ores surface features, this reflects the ability of nomic deposits are highly varied (Tables 1 and 1978; Witschard, 1984). These suggestions were the result of hydrothermal and metaso- presently are speculative. As no critical evi- erosion to remove the topographic and geologic 2). In the simplest terms, the types of deposits matie action, which occurred during the Lower dence for the occurrence of impact, beyond expression of impact craters. Some of the recog- can be classified according to their time of Proterozoic, on ferruginous quartzites (jas- general statements regarding "circular struc- nized craters, however, arc exhumed structures, formation and are discussed below under the pilites) and some other lithologies, producing tures," has been presented, they are beyond the having been buried by post-impact sediments classification: progenetic, syngenetic, and epi- zones of albitites, aegirinites, and, amphibole- scope of this paper. and only relatively recently re-exposed at the genetic (Table 1). Progenetie economic deposits magnetite and carbonate-hematite rocks (along surface. If observations are restricted to large, are those that originated prior to the impact with uranium mineralization), which now are young craters on the well-searched cratons, event by purely terrestrial concentration mech- exposed in the crater floor. Post-impact hydro- then the terrestrial record indicates a cratering Progenetic Deposits anisms. The effect of the impact event has been thermal alteration led to the remobilization of rate of 5.4 ± 2.7 X 10•ts km2 a-t for D >_ 20 km to spatially redistribute, with respect to the Progenetic economic deposits in impact cra- some of the uranium mineralization and the (Grieve, 1984). The large uncertainties ters include iron, uranium, gold, and other surrounding area, the lithologies containing formation of veins of pitchblende. The product attached to this estimate reflect concerns over minerals (Tables 1 and 2). In many cases, the these deposits arid, in some cases, bring them to tion of uranium ceased in 1967, but iron ore is completeness of search and crater recognition. deposits are relatively small. Here, we consider a surface or near-surface position, where they still extracted from two main open pits— This rate is very similar to that calculated from only the larger and more active deposits. can be exploited. Syngenetic deposits are those Annovskiy and Pervomaysk. The total reserves astronomical observations by Shoemaker et al. that originated during the impact event, or at the Pervomaysk open pit are estimated at 74 (1990) for the present rate of impact by Iron of Ternovka immediately afterwards. They owe their origin million metric tons, with additional reserves of asteroids and comets and is equivalent to 4 ± 2 to the energy deposition of the impact event Iron and uranium ores occur in the basement 20 km being formed lower-grade deposits estimates) at approximately impact structures with D>_ in the local environment, resulting in phase rocks of the crater floor and in impact breccias on the land surface of the earth every 5 million 675 million tons. As a result of brecciation and changes and melting. Epigenetic deposits result at the Ternovka structure, Ukraine (Nikolsky et years. displacement, blocks of iron ore are mixed with from hydrothermal alteration, the formation of al., 1981, 1982).1'11e structure is 375 ±25 m.y. barren blocks. These blocks are up to hundreds an enclosed topographic basin with restricted (Nikolskiy, 1991) and was formed in the Lower of meters in diameter and are "rootless," hay- sedimentation, or the long-tens flow of fluids Proterozoic fold belt of the Krivoy Rog iron ore ing been rotated and displaced from their pre- General Character of Economic Deposits into the structural trap formed by the impact basin. It is a highly eroded complex crater. 'Elie impact positions by hundreds of meters. This The location and origin of economic deposits structure. • parautochthonous rocks of the crater floor are displacement and mixing of lithologies causes in impact structures are controlled by several There have been suggestions that deep-seated exposed and there is a central uplift and annu- operation and evaluating the factors related to the impact process and the faults associated with ancient large impact lar trough (Fig. 12). "llte central uplift is, in difficulties in specific nature of the target. A; impart struc- structures have acted as controls for the endo- part, brceriated and injected by dikes of impact reserves. On the other hand, impact•induccd tures can occur on any type of target rock and genic deposition of various metals frurn depth melt rock tip to 20 in wide. The annular trough fracturing aids in extraction anti processing.

1 7 Z ~, 1 _ 1 yuanutns, at,nlst5^ 4 4 1 disturbed rocks carbonate rocks ^ I t `'` • •. 4 I •; • !• • • .•.• • • I• + ,'• 4 4~4 III •/• • • t • o-• ♦ •• 1 1 I 1 I 4 M+ 4 I 1 • • I • • • ++ + + 4 4 + + • ` +' 1 1 • + C 4 • + •+ + + +♦ I 1 1 • 4 4 + • • + + 1 • • ` + 4 4 + +++44 + + 4•+ 4 + 4 4 • 4 ••ti 4.^ • + + • I 4 IVI 4 • + Y Y u V V V_ V_ V Y ~• 4 + + + • + 4 + • C C C C C C C C C C C C C C C C C C C Ÿ Ÿ V li Ÿ_ y Ÿ Ÿ + + + + 4 + + + + + g$ C C C C C C C C C C C C C C C C + + + + . t =`=°~c éommmYo~ommâom éoSô~m^O°o" < 4 A 4 ââ?'$ a~~+ mââ rOrm. 0 1 I,Q -/ L 1 L .1 wwaw w ~ o.aiww ai/naititit~rn vt t$r$ I.~i wt~saiwwâwlnâ Lut tnâ ere en. c n < n < n c c 9 nn >~ n ~~ ~n >~ n > - nn A 4 A<4 A < L A< I.< A < Ln,c~ Lnac amphibolites Ln'~ n >< n >~ n> < < ;A < 4A A <4A ~4 .Y.■ ~ 4 L a 1 L ~1 c J n < J n < + n > h O 4 I 4 + 4 -c2 E Z _3 ++ +++4 Y ~ : U i 1 L ` +/ + + + + + 4 ~ - • / + + + + + ,+ L .O + + + + + ~ ôé?• ~ ~ tg r• — qû E E E. Lû 2 a u „ ` e c ~ ~ W + + + + • v + 1 L,J 1 f + + —~EY 4131 ~aY•ev~ P Y ;11 c c w ~w ~E < + + 4 granites {. 1 t û ÿ_n• ka gag: J n `•*-Éc°°Y~gÉgNgâ.Ei ♦ + I • + 1 4. 4 4 w i mvô—ôg-mcÂ~ A I. A + + + } + + 4 + + + • ë .3 a = 3~esc>o_â:~o—~~cb~c~~ .3 ..3bU=ôc>6rg • jaspilites, schists ° L a 1 L 4 + t .1 4 1 4 + n < + 4 +++ + I n >, 4 f 4 1 4 t + • + + +• • 4 4 4' < + + + + • + + 4 ++ + a 1 t-- a • f + } 1 • + + < J n c à + + I • 1 2km ~ + I I ,r > ++ + + + + I t 1 f + cv ,n co t- .n < 433 7 1 f • • 4444444 I A J n_ ~ C ,_ O . . ~ • ^ O8 ~NpNn Np CI ^ M

CU t Central Uplift Allochthonous Breccia

Open Pits g Block of Jaspilite

0003000 wawwwawmww140w3w a a atalw wwww Fu:. 12. Geological sketch map of the floor of the Ternovka crater. Oren pigs, from b.~nnb~r ~y~ô co which iron ore is ~ ô éô b ~v extracted, are indicated. pppp ~_ PNM"1 i0 CC M_ ~ G ÇNGG - M OI p 7. 7T T 7. .. _ 7e~ tl T -l beyond allochthonous dike-like and other ,n .p tp o Uranium at Carswell N.+. ~ Ô ~ V N M bN V.Ji O +f ~O( ~ ✓Al r a g é Ô~M ~ ~ N • 0 0000000* 0000 00000 •01,41 0 bodies of breccia and impact melt rock. The MM1 NM1 p Uranium ores also occur at the Carswell V 2 ~ ~ complex structures—e.g., Charlevoix. (Rbndot, i9: 2 cs.~ tt generally is quoted as the diameter of the struc- ~ ô =d ~ 7p , ï -N ~if ~~ c > ~ a ~ ture. There is an annular trough, which is about 1972), Manicouagan (Grieve and Head, 1983), yq ~ = 5 ~ „ E S km wide and rises to a core, roughly 20 km in Siljan, Sweden (Rondos, 1975; Grieve,1988)— ~ ô o . ~1 mtl. gq hi LO 7 in :7 i lV iV diameter, of metamorphic basement (Fig. 13). with preserved inliers forming a ring of down. Essentially no crater-filling materials remain, dropped lithologies, the original diameter of the

rper ttoy, nett et at. t t•soor,:MU i age' tutu tures, coesite, stishovite, find ëvidencc for high uppermost units of the Athabasca Group. Their dipping to overturned Proterozoic sedimentary ab (1985). The basement core is believed to temperature annealing (White,1993; Leroux et outcrop is unique to the area and they owe their and volcanic rocks of the Witwatersrand and ve been uplifted at least 2km (Harper, 1982). al., 1994). These recent developments, plus preservation to having been downfaulted at Ventersdorp supergroups approximately 18 km rrounding the core arc units of the other ongoing work at Vredefort, have been metamorphosed Athabasca Group: sand- least 1 km to their present position (Harper, wide, and an outer broad synclinorium of vol. sufficient to convince some of the previous mes and conglomerates of the William River 1982; Tons et al., 1985). gently dipping Proterozoic sedimentary and sequence ~ 28 km antagonists that Vredefort is the site of a major • hgroup, sandstones and silistones of the than' mineralization is concentrated ranic rocks of the Transvaal wide. The southeastern portion is covered by impact event that occurred --~ 2 Ca (e.g., l Iart et luglas Formation, and dolomites of the Cars. along the south to southwest contact between younger sandstones and shales of the Karoo al., 1991; Gibson et al., I994). The rnetanor- 11 Formation. The inner contact with the the basement core and the Williams River Sub- ?2 R. A. E GRIEVE AND r' L AfAtiA111.S TF:RRFS'TRIAI. IMPACT CRATERS 123

F group, with an estimated 46,500 metric tons of sequence (Fig. 14). 'Die general circular foam 109°IS'~ titanium originally being present in the cur- with an uplifted central core, the occurrence of rently known ore deposits (Harper, 1982; writ- stishovite arid cocsite, arid planar deformation ten communication, 1994). Mineralization features in quartz and shatter cones all have occurs in both the basement core and the been presented as evidence that the Vredefort Williams River Subgroup (Fig. 13). The original structure is the eroded remnant of a very large, mineralization is believed to reflect regolith complex impact structure (e.g., Dietz, 1961; development under tropical lateritic weathering Hargraves, 1961; Carter, 1965, 1968; Manton, of the basement prior to deposition of the 59°0' — 1965; Martini, 1978, 1991). The impact inter- Athabasca Croup. The main exploited mineral- pretation has been challenged (Hart et al., ization resulted from a hydrothermal event at 1981; Schreyer, 1983; Winter, 1986) and even 1100 ± 50 Ma, which produced a uraninite- the existence of bona fide shock metamorphic polymetallic sulfide assemblage. Variations in features has been questioned (Lilly, 1981; the mineralogy of the ores are related to the Simpson, 1981; Reimold, 1990). The problem type of host rock. Additional periods of at Vredefort is that, as the center of the struc- remobilization also occurred, producing pitch- ture is approached, the planar deformation fea- blende and a number of separate parageneses tures in quartz die out (as opposed to increasing for the ores. Details of the ore deposits and their in number and occurring in differing orienta- origin can be found in papers in Lainé et al. tions—representing increasing levels of (1985). Like Ternovka, the relationship recorded shock pressure—as at other large bet ween the ores and the Carswell impact struc- complex impact structures) (e:g., Fig.• 10) . This ture is not a genetic one; rather, it was the unusual behavior is attributed to increasing physical uplift of the basement core by at least 2 levels of recrystallization of quartz as the center km, during the formation of a complex crater, is approached (Grieve et al., 1990). Another that brought the ores to their present high Ar Ore Bodies 0 km to unusual feature is that the planar deformation I t J levels, where they could be recognized and — Major Fault features are dominated by so-called basal fea- 5a°1 exploited. In addition to mechanical mixing tures, which are the planar deformation fea- Carswell Formation - Dolomites due to brecciation, minor rernobilization of ores ~-~ tures produced at the lowest pressures (>7 occurred, following the impact with the produc- Douglas Formation - Sandstones, Siltslooes HGLINIAN CPa) (Robertson and Grieve, 1977; Stüffler and tion of a coffrnite-sulfide mineralization in the langehorst, 1994). William River Subgroup - Sandstones, Conglomerates breccias (Ruh6nann, 1985). Recent work on planar deformation features Metamorphic Basement - Grannoids, Gneisses ARCHFAN Gold and Uranium of Vredefort in quartz using a transmission electron micro- scope (TEM) has indicated that there is a funda- Cold has been mined intermittently in the mental difference in the nature of basal planar FIG. 13. Geological sketch map of the Carswell crat-ir. Locations of known uranium orehodies are. collar rocks surrounding the so-called Vredefort indicated. Dome, with the main mining occurring in an deformation features and other orientations; namely, the basal features are the result of area known as the Venterskroon or Rooderand ,rswell structure may have been ;n the 50-55 basement core is, in places, faulted and trun- Goldfield (Holland et al., 1990). The origin of mechanical Brazil twins, whereas the others, range. cated and offset by radial faults. The outer Vredefort has been controversial (e.g., see when fresh, are lamellae of a mixture of silica The basement core consists of mixed feld- contact of the Carswell Formation also is char- Nicolaysen and Reimold, 1990), with several glass and crystallites (Goltrant et al-, 1992; ithic and mafic gneisses of the Earl River acterized by arcuate faulting, drag folding, and workers in S. Africa attributing a cryptoexplo- StSffler and langehorst, 1994). This funda- .mplex, overlain by the more aluminous Peter local overturning of beds, as well as offsets by sion origin to the structure. The structure is mental difference results in quite different ver gneiss. Details of their mineralogy and radial faults (Fig. 13). There also are a number generally taken to consist of an uplifted central responses to thermal annealing. In the casé of emistry can be found in Herring (1976), of regional faults unrelated to the structure. core of predominantly Archean granites Vredefort, TEM investigations have confirmed lrper (1982), Bell et al. (198.5), and Pagel and The Carswell and Douglas formations are the roughly 44 km in diameter, a collar of steeply the presence of basal planar deformation fea- ab (1985). The basement core is believed to uppermost units of the Athabasca Group. Their dipping to overturned Proterozoic sedimentary tures, coeslte, stishovite, and evidence for, high ve been uplifted at least 2 km (harper, 1982). outcrop is unique to the area and they owe their and volcanic rocks of the Witwatersrand and temperature annealing (White, 1993; Leroux et rrounding the core are units of the preservation to having been dowttGntlted at Ventersdorp supergroups approximately 18 km al., 1994). These recent developments, plus metamorphosed Athabasca Croup: sand- least 1 km to their present position (harper, wide, and an outer broad synclinorium of other ongoing work at Vredefort, have been mes and conglomerates of the William River 1982; lbna et al., 1985). gently dipping Proterozoic sedimentary and vol- sufficient to convince some of the previous bgroup, sandstones and siltstones of the Lira mineralization is concentrated canic rocks of the Transvaal sequence 28 km antagonists that Vredefort is the site of a major ruglas Formation, and dolomites of the Cars- along the south to southwest contact between wide. The southeastern portion is covered by impact event that occurred — 2 Ga (e.g., Ilart et II Formation. The inner contact with the the basement core and the Williams River Sub- younger sandstones and shales of the Karoo al., 1991; Gibson et al., 1994). The niet amor- 71:71R6:STR14I. IMlt4(:T (:lt.•1TTi1(.ti IZS

T

km t!)0

26_

V v V V V V V V V V V V V V V V V ~V V VVVVV V V V V V V V V V V V V V V V V V V V V V V V V V V V V V 2r 2r- ,TvvvVv~VVV ✓~V [- Karoo Sequence REDEFORT 7 V V V~ V V V V V V V vvv VVV~VVV~VV vvv v vv ✓vvvV m nTransvaal Sequence

Itism is the result of the intrinsic heat of the the center is roughly 36 km, then the relation• IS. Geological sketch map of the Vredefort structure, with the partially overlying Karoo, Sequence )cks of the core at their original depth plus ship for the amount of structural uplift in the removed. Indicated are the series of concentric antiform and synform struct tires surrounding and related to )me post-shock healing (Grieve et al., 1990; center of complex impact craters given earlier Vredefort. Saurce: After McCarthy et al. (1990). art et al., 1991; Martini, 1992; Gibson et al., would indicate that the original diameter of the 994). Recent work also has confirmed that a Vredefort impact crater was on the order of 335 km, although the relationship shows consider- downdropped within the annular trough exte- able scatter. rior to the central uplifted core of a very large, tughly 36-km section of the crust is exposed at km. Using arguments involving the relative spa- With an estimated original diameter of probably originally multi-ringed, impact redefort and that the entire structure has been tial distribution of shock metamorphic features roughly 300 km and an age of about 2.0 Ga structure. lied to the northwest by post-impact tectonic and downfaulled outliers in the annulus sur- (Walraven et al., 1990; Spray et al., 1994), the' The gold fields of the Witwatersrand Basin Tees (Hart et al., 1991). In addition, Re-Os rounding the central core, developed originally otopics on the so-called bronzite granophyre, Vredefort structure encompasses a series of are a geologic wonder, having produced over to reconstruct the dimensions of the Sudbury hich occurs in dike-like bodies at Vredefort post-Transvaal anticlinal and synclinal struc- half the gold ever mined! They have been mined Structure, Therriault et al. (1993) estimated td has been interpreted as the remnants of the tures surrounding the central core, identified for over 100 years, with total production, as of that the rim of the transient cavity at Vredefort ipact melt rocks at Vredefort (French and by Simpson (1977) and McCarthy et al. (1990) 1985, being over 40,500 metric tons and a value' was 100 km in diameter, at the present level of ielsen, 1990), indicate an admixture of mete- as being related to Vredefort (Fig. 15). More on the order of $50 billion (Pretorius, 1986x). erosion. When the amount of erosion (^-8 km) itic material (Koeberl, oral communication, importantly, McCarthy et al. (1990) also note Associated with the gold is uranium, with total is taken into account (Therriault et al., 1993) 194) . that if it were not for the preservation effects of production, as of 1985, of over 136,500 metric and the transient cavity is scaled to final crater With a greater understanding of Vredefort, these structures concentric to the core of the tons, with a value on the 'order of $4 billion d its somewhat complex geologic history, has rim, the original rim diameter may have been on Vredefort structure, much of the Central Rand (Pretorius, I986a). Current revenues .from me a reassessment of its original size. Pre- the order of 300 km. If the cenier of the sur- Group would have been eroded from the area mining of gold and uranium are of the order of us estimates for the original diameter were rounding Potchefstroom synclinoriurn is taken (Fig. 16). The major gold deposits of Witwaters- $7 billion p.a. There is a massive literature on the order of approximately 140 km (Grieve, as the beg' g of the central uplifted area, the rand Basin occur in an — 180° are at the outer the ores of titi- Wit watersrand Basin and it is riot 91). They were based on the thickness of the central uplift area has a diameter of ca. 75 kin edge of the outcrop of the Central Rand Group our intention to review it here (e.g., see ;Pile- lifted section exposed in the collar rocks and (Fig. 15). From the relationship of diameter of (Fig. 17). The situation differs from that at, for torius, 1976, 1986b and references therein). I not take account of the 22 km crustal central uplifted area to crater diameter illus- example, Carswell, where the ores are exposed The uranium is iu the form of uranite and is lion exposed in the central core (Hart et al., trated in Grieve (1991), the original riot diame- because of uplift in the center. At Vredefort, the detrital in origin, with limes of formation

1 Lambert, t7oi,- t tiese retain/Os met ety tttur- Ries, Ternovka, Zapadnaya, 10 documented evidence of the occurrence of turc with original dimensions on the order of Puchezli-Katunki, cate generation during different phases of the and others (Masaitis, 1993a). lck metamorphic effects at the Bushveld 300 km. cratering event. Claims that •t^Ar/39Ar dating (-ench, 1990).

R. A. EGRIEVE. AND l' L. MA.SAITIS TERRESTRIAL IMPACT CRATERS 1211

Impact diamonds originate from phase Iran- with an individual impact structure and within tions from graphite or crystallization from individual impact melt and suevite bodies can al, which originally has been shocked to a be irregular. They tend to be. concentrated in laid or, possibly, gaseous state. They occur rays or zones emanating from areas where the hen their precursor carbonaceous lithologies original carbon-bearing lithologies are most ;re subjected to shock pressures ? 30 CPa abundant (Masaitis, I993a). They also tend to lasaitis, 1993a). The diamonds from graphite be radially restricted. Closer to the center of the crystalline targets usually occur as para- structure, high post-shock temperatures cause orphs, with inherited crystallographic fea- more rapid oxidation of the diamonds. On the rer (Masaitis et al., 1990, Val'ter et al., 1992). other hand, at some critical distance from the iey occur generally as microcrystalline aggre- center, the shock pressures are too low to result tes, which can reach 10 mm in size, consisting in the phase transformation to diamond. Chill- cubic diamond and lonsdaleite, with indi- ing of the impact melt aids preservation. dual microcrystals of 10-4 cm. The diamonds Although diamonds associated with known oni coal, or from other carbon in sediments, impact structures are not currently exploited nerally arc porous, colored, and may have a commercially, those produced by shock trans- limpsest biogenic texture, i.e., they have formation of graphite have some technical me of the structural properties of the original advantages over normal diamonds in that they ologic matter. tend to be harder and more resistant to Diamonds occur in a variety of breaking. lochthonous lithologies at impact craters. Carbonados are irregular polycrystalline dia- ley are most common as inclusions in impact mond aggregates that occur in placers and in sit rocks and in impact melt clasts in suevite low-grade m¢lamorphic rocks. They occur in eccias. For example, at Zapadnaya, Ukraine, Brazil, Russia, South Africa, Ukraine, Venezu- ey occur in impact melt dikes in the central ela, and the Central African Republic, where 'lift and in suevite breccias in the peripheral they are the main export for industrial diamond nigh. Zapadnaya, -- 3.8 km in diameter and applications (Treuh and de Wys, 1971; 5 ± 10 m.y., is formed in Proterozoic granite Kaminsky et al., 1978). Carbonados are associ- ntaining graphite (Gurov et al., 1985). At ated with crustal parageneses and arc not tpigay, Russia, the allochthonous breccia fill- related to kimberlites. They have a range of SI3C g the peripheral trough is capped by diamond• values that overlap those of the diamond-lons- aring suevites and coherent bodies of impact daleite aggregates in the Ukrainian placers -.It rocks (Figs. 18, 19). The largest of these (-15.8%i to -20.5%«)(Kaminskiy et al., 1977) It ..It-rock bodies can be traced for 10-I5 km .. + ~1J 14%0 XA1 Vll and the diamond-lonsdaleite-graphite aggre- 1 2 3 4 5 6 7 8 9 10 mg strike and is 500 m thick (Masaitis et al., gates in the suevite breccias at Popigay (-12.3%n, 80). In the case of Popigay, 100 km in diame- to -17.6%0n) (Galimov et al., 1980). In some Plc. la. Geological sketch rna11 of the Popigay crater. Legend, I = Archean schisis and goeisses; 2 = Upper and 35 ± 5 m.y., the original source of the rises, lonsdalcite also is present in Proiemzoic and Cambrian quartzites, dolomites, and limestones; 3 - Perna. sandstones; I = irnpacl inch rbon is Archean gneisses with graphite, carbonados rocks; 5 = suevites with lenses of clastic. allochthonous breccia; 6 = allochthonous breccia; 7 = uplifted area; rich are overlain by up to 1.5 km of Pro- ar,d they generally are believed to be Pre- 8 = axis of annular trough; 9 = radial troughs; 10 = faulty, thrusts. -ozoic, Cambrian, Permian, ard Cretaceous cambrian in age (Kaminskiy et al., 1978). The -rigenous and carbonate sediments. The dia- origin of carbonados is debatable but the iso- have been eroded away, but the carbonados peting hypotheses for the origin of carbonados mds at Kara, Russia also occur in impact melt topically light character, noble gas contents survived the subsequent physical and chemical (e.g., Kaminskiy, 1987; Kagi et al., 1994). cks. Kara, 65 km in diameter and 67 ±6 m.y., indicating trapped atmosphere (Ozima et al., 1991), rare earth abundance patterns (Shibata activity and became incorporated in later sedi- located in a Paleozoic fold belt, in which Cu-Ni sulfides at .Sudbury rmian terrigenous sediments contain coal et al., 1993), and associated parageneses indi- ments and low-grade metamorphic rocks, in zerskii, 1982). cate a crustal source. some cases, passing through the sedimentary The Cu-Ni and PCM ores at Sudbury are In impact melt rocks al structures with ear- 'l'he wide variety of parageneses between car cycle several tunes. Apart from the lonsdaleite, world class, with an estimated 1.65 x 10" metric n-hearing lithologies, diamonds can occur in bonado deposits suggests a variety of progeni- there is little other evidence of shock in associ- tons of 1.2% Ni and 1.05% Cu; they are related ry minor amounts, with provisional average tors. This range of sources and the presence of ated minerals, but Ibis could be a result of the to the Sudbury Igneous Complex (SIC). Over Iimateson the order of 10 pph. When the melt lonsdaleite led Smith and Dawson (1985) to age of the carbonados, the effects of annealing, the last five years, the extraction of these ores itrix has cooled relatively slowly, the dia- suggest that carbonados were lirmed by Pre- and tepr-ated recycling through the sedimen- has had au average value of $2 billion per mds show evidence of reversion to graphite eanlhrian-aged impacts into carbon-hearing tary cycle- At the present lime, however, impact annum. The origin of the SIC has been contro- rl oxidation. The distribution of diamonds lithologies. lice ilscur-lated I,rr-re ras long since must be considered as oils one of several row. versial and :1 considerable him:tture exist'.

10 Y,. A. F. GRIEVF. AND V I.. MA.SAITI.S TFRRF.:STRIA1. IMPACT CRATERS 131

x _~c P • • ~ :• 5`~J G...., Q4w

GRENVILI.E PROVINCE .eha

7-7 Sudbury Igneou5 li—I Crimple/ —4.1 •-•_ • -wnnewale, Senes • . , ÿ~<: • _ .. ® Sudbury fl,eor,a .• SMne, Cones • outs, ',mill .e.00 LAKE HURON km Ico taYoa lkm I B IBOm A I g FIG. 20. Geological sketch map of the Sudbury Structure and its environs. Note occurrence of Sudbury Breccia at considerable distances from the Sudbury Igneous Complex (SIC) and the distribution of shatter Gneisses Suevites Impact melt rocks Overburden cones outside the SIC, indicating that the Sudbury impart structure is murk larger than the outcrop of the otiè SIC. Source: After Peredery and Morrison (1984). Q o 'b ~~: and its ores by impact, followed almost immedi- interior Sudbury Basin (Cihlin, 1984). The Ftc. 19. Geological sketch map of Udarone impact diamond deposit at Popigay. Diamonds occur in the melt ately by extensive deformation due to the Sudbury Structure covers currently an area of rocks and melt glasses of the suevite. No vertical exaggeration in the cross-section. Penokean orogeny. It is not our intent to review >15,000 km2 (Fig. 20)-.'flic outcrop of the SIC this recent literature here, except to point out is not synonymous with the Sudbury Structure. kerning aspects of Sudbury geology (e.g., of the impact process, the voluminous nature of some salient conclusions. For a more complete The SIC is only a part of the Sudbury Structure. lees in Pye et al., 1984; Lightfoot and the SIC and the fact that it was differentiated development of the more recent arguments and The Sudbury Structure is superimposed on a ldrett, 1994; and references therein). Sud- into lower noritic-gahhroic and upper grano- conclusions regarding the impact model, the Proterozoic supracrustal sequence (Huronian -y was considered the site of explosive volca- phyric units posed problems for a simple impact reader is referred to Grieve et al. (1991), Supergroup), overlying Ar'chean basement m combined with intrusive igneous activity, origin. As a result, even proponents of the Deutsch and Grieve (1994), Grieve (1994), and rocks (Abitibi Subprovince of the Superior impact origin suggested some combined impact it Dietz (1964) suggested that Sudbury was Stüfflcr et al. (1994). This is not to say that Province). The Proterozoic metasediments and and magmatic models (e.g., French, 1970; site of a major impact, based on the occur- there is complete consensus and understanding metavolcanics are exposed mainly in the south , Dressler et al., 1987). of all aspects and details of the impact origin of ce of shatter cones. Since then, other physi- and east, whereas the basement, rocks to the Recent advances, however, in understanding the Sudbury Structure and the SIC (e.g., Chai north and west are Archean Levack gneisses evidence of impact in the form of other the relative amount of impact melting at large and Eckstrand, 1993; Chai et al., 1993; Mas- bordered by granite-greenstone.lithologies. To ,ck metamorphic effects has been described impact structures (Grieve and Cintala, 1992), aitis, 1993h). the north, erosional remnants of the Huronian he basement of the so-called Footwall rocks, as well as geologic, geophysical, and geochemi- Despite the voluminous literature on the Supergroup also are fot`utd as outliers in a ring- he Onaping suevitic breccias overlying the cal studies at Sudbury (e.g., Faggart et al., 1985; geology of the Sudbury region there is often, like graben zone parallel to the boundary of the and the so-called Sudbury breccias in the Deutsch et al., 1989; Lakomy, 1990; Shanks and still, a basic misunderstanding as to what SIC at a distance of 20 tu 25 km (Fig. 20). ,twall rocks (e.g., French, 1968; Peredery, and Schwerdtner, 1991a, 1991h; Walker et al., constitutes the Sudbury Structure. The. Sud- Formations related directly to the Sudbury '2). The major stumbling block in under- 1991; Dickin et al., 1992; Milkereit et al., 1992; bury Structure is a collective term for: the Structure occur up to 80 km away from the iding Sudbury as an impact structure has Hirt et al., 1993; Avcrmann, 1994; Cowan and brecciated country rocks of the Superior and outer margin of the SIC (Fig. 20). The following n the presence of the SIC and the elliptic Schwerdtner, 1994; Deutsch, 1994; and others) Southern provinces of the Canadian Shield sur- formations, proceeding inwards and strati- ure of its outcrop (Fig. 20). Although impact point to a relatively consistent picture for the rounding the Sudbury Igneous Complex, the graphically upwards through the Sudbury ling was recognized early as an integral part origin of the Sudbury Structure and the SIC SIC itself with its associated ores, and the Structure, can be identified: (I) dike breccias 132 R. A. FGRIFVF.4N!r 1: 1.. MA.SAITZS T'F.RRFSTRIAI. IMPACT r:RATI•:R.S 133

Impact - magmatic Lithology Impact Interpretation impact interpretation arc contained in the pre- gests Thal the original diameter of the present mudslone viously cited references. The major conclusions exposure of the SIC could have been at least 65

Upper Plank Member of these studies can be synthesized as: (I) all kin and, perhaps, a-' much as 75-80 km in its - reworked clastic maters Mecca "polyrnict" formations (breccia and melt pre-erosional form. A similar conclusion is

Iowa, Black Member lithologies) can be explained by impact- reached by Hirt et al. (1993), who removed the

Green Member- Ilalback'I RECCIA induced brecciation or melting, mixing, and strain in the Onaping and Chelmsford Forma-

TE B transport of Precambrian basement rocks, tion, based largely on magnetic anisotropy fab- T

EV which now are exposed at the Sudbury Struc- Using the same empirical relations as Gray Member- rics. SU ture; (2) there is no chemical or physical previously (Grieve et al., 1491), this would rWound «av.-I requirement for additional "volcanic-mag- extend the maximum diameter of the float cra- matie" components derived from the mantle ter rim to some 250 km (Deutsch and Grieve, Mixture Impact men bode, 1.85 Ca during the Sudbury event in any of the 1994). Regardless of the detailed determination and suevne brecc,a formations of the Sudbury Structure, in partic- of the original size, the basic conclusion- is that ular for the Sudbury melt system (the SIC, the the Sudbury Structure represents an' eroded parr-rIth upper nice Basal Memher of the OF, and melt particles in and heavily tectonized remnant of an originally various other breccia units) and the ore 200- to 2.50-km-diameter peak-ring, or possibly deposits. a multi-ring, impact basin: The amount

T of ero- The interpretation of recent seismic reflec- sion at the Sudbury Structure is generally HEE tion data indicates clearly that the SIC has been

T S unknown. It generally is estimated at a few highly tectonized in the south and subjected to kilometers (Naldrett and Hewins, 1984; Low• T MEL NW-SE shortening as a result of the Penokean man, 1992), certainly less than at the similarly PAC orogeny (Milkereit et al., 1992; Wu et al., M I aged and sized Vredefort Structure. U-Pb ages 1994). This picture of northwestward thrusting of new zircons from the SIC, the immediate is consistent with recent structural observa- Footwall I.evack Gneisses, as well' ds shocked tions that indicate ductile deformation was fol- zircons from the Onaping, all indicate an age of lowed by thrusting (Shanks and Schwerdtner, 1.85 Ga for the Sudbury event (Krogh et al., 1991a, 19916; Cowan and Schwerdtner, 1994). 1984). Thus, the present configuration of the SIC is In general, many of the orcbodies ai Sudhury the result of tectonic modification. In acknowl- are located in what are termed focally as edgement of the role of tectonism for the pres- Clnst rich Basal Meir "embayments," which may, be related to ter- ent geological and geophysical character of the races in the original impact crater (Morrison, lhermaay metamorphnsM Sudbury Structure, finest and Pilkington elastic Mec

OO (1994) reconstructed the original spatial rela-

mnnommt b,ecGas FL deposits at Sudbury occur in a variety of asso-

R tions of the potential field data by removing the ciations. Naldrett (1984) recognizes five major TE effects of a simple NW-SE stress field. Their

dike brep:ias RA associations: (i) South Range, where the ores (pseudoiachylnes) C conclusion is that the anomaly pattern of the D occur largely as massive sulfide' deposits at the

TE present SIC could well have been originally A circular. hase of the Sublayer; (ii) North, Range, where e' CCI the ores occur largely as concentrations of

RE StSffler et al. (1989), Lakomy (1990), and B Grieve et al. (1991) presented estimates of the sulfides (up to 60%) in the Footwall Breccia and dimensions of the original Sudbury Structure. as stringers in the Footwall; (iii) offset dikes, These estimates were derived by analogy with where the ores occur 11argely as concentrations Fie. 21. Stratigraphie column at the Sudhury Structure with previous and current impact genetic empirical relations at other large impact struc- of sulfides in breaks and constrictions in the I It terprelat mit. tures between the observed radial extent of dikes; (iv) faults, where the ores occur as various levels of shock effects, brecria layers, remobilized masses; and (v) other associations. and pseudotar•Itylite (collectively named Sud- The interpretation of this stratigraphie suc- dike breccias, and specific morphological and The association of the ores is cot l telex, varied. bury Brerc-ia), (2) shocked foot wall rocks, (.') cession in terrils of current understanding of structural clt niertis of impact st flirt tires. These and, in suint. cases, exceptional. This variability Foot wall Ilreccia, (4) Cottart Sublayer anc. the formation of a large complex impart crater istirmnes of the diameter for the original final reflects complications resulting front thr'rntal Sublayer-Offset dikes of the SIC, (5) Main Mass differs from prey' views that considered an crater and the transient cavity depend strongly metamorphism, including partial melling of of the SIC, (6) Basal Member, Onaping Forma- rndugenic nr mixed eudogen ic-exogenic. origi ii on the original diameter of the SIC at the the Footwall, and hydrothermal activity Ihat tion (0F), (7) Gray Member, OF, (8) Green and is presented in Figure 21. Details of the present erosional level. lite " drstraining'' of occurred because of the extremely hot SIC and Member, OF (9) Black Member, OF, and (1(1) basic geologic, petrographic, geochemical, and the pattern of potential field anomalies related elevated post•intpact temperatures in the Foot Onwatin and Chelmsford formations. isotopic studies litat have led to this general to the Si(: (Mies! and Pilkington, 1994) sug- wall brceuse of shock and uplift. More imppi

134 R. A. F. GRIEVE ANI) V L. MASAITIS TERRESTRIAL IMPACT CRATERS 135

within the general framework of the formation •Archean Granites •1 ~~' •:. Mantle (0.1) ; -t of a 200- to 250-km impact basin 1.85 Ga, with Archean - Huronian rocks and volcanics :~;.. :..::.:'?;;;•:~ .~•::a n•et •' ot .~: c -~•'. ••• accompanying massive crustal melting produc- u II I III { ii ?1.‘• ttz'-:, :~:: ~..Mt.Vtn~.• ~ ing a melt of an unusual composition that gave ;•tik~ rise to immiscible sulfides and ultimately the ( OSJreaOS)i - i es ca ~.~e:~.':~'•'VO+~o~S ;: i" present ore deposits. A complicating but essen- •+r~Y ;~; ' \ •~ ; ; •~ •; t , ~ ~tiw el, A ~., tial component of the evolutionary history is the almost immediate deformation by the pene- Sulphide Ores of Sublayer SIC :?~::t? . ,if contemporaneous Penokean orogeny. Although D.:_t..-...;.;» .,,.. ••.~i ~~~lf explanations involving massive crustal assimila- f_. tion by magma (e.g., Walker et al., 1991) or "unique igneous activity" (e.g., Chai and (a) l Eckstrand, )993) still are forthcoming, it ~ 0.2 0.1 0.4 0.5 0 6 • should be noted that even in these cases an impact event is required to trigger the activity. ,••.t.••:•.t 1••:•.•• Thus, impact still remains an essential compo- nent for the genesis of the Sudbury ores.

Epigenetic Deposits Epigenetic deposits generally reflect the fact that impact structures can result in isolated Post impact Sediments Impact Melt System • Ore bodies (Onwalin. Chelmsford f.) 1.31.+Al (SIC. Basal M., Sublayer) topographic basins or locally disrupt under- Silicates of Sublayer SIC ground flow. They can originate almost immedi- Suevite (Grey, Green Archean high-grade and Black Member, Levack Gneiss ately or over an extended period after the Onapirg F.) impact event. Examples include various hydro- thermal deposits, liquid and gaseous hydrocar- E T — I.xs Ga FIG. 22. Location of main orehodies at Sudbury. Note their spatial relation to the base of the impact melt bons, oil shales, various organic and chemical Nd system (SIC) (see text for other details). sediments, as well as flows of fresh and miner- (b) alized waters (Tables 1 and 2). tantly, the formation of the Sudbury Structure initial composition that already was enriched in j0 and its almost immediate deformation by SiO, (-64%) compared to endogenic magmatic h ydrothermal ores +5 (t -5 Penokean folding and, later, thrusting (Cowan compositions. Additional assimilation of clastic. Hydrothermal ore deposits occur at Siljan, and Schwerdtner, 1994; Deutsch and Grieve, debris may have abetted this unusual composi- Ftc. 23. Histograms of calculated osmium isotopic data Sweden (55 km, 368 ± 1 Ma). Here, Pb, Zn, and 1994) gave rise to complex post-impact move- tional character and, indeed, assimilation of for sulfide ores and country rocks at 1.85 Ca (A) and Ag sulfides occur in veins in the contact zone ments and mobilization. local clastic debris may account for some of the calculated neodymium isotopic data for silicates (ran- 5ûb- between sandstone and limestone blocks in the layer ores and country rocks at 1.85 Ca (13) at Sur bury. The overall commonality to all the ore compositional differences between the North annular trough outside the central uplift. The Source: After Dickin et al. (1992). deposits is that they lie at the base or just and South ranges of the SIC (Naldrelt et al., amount of ore is estimated at 300,000 metric beneath the SIC (Fig. 22). In previous magmatic 1984) . Ag. These types of deposits are related to hydrother- models of the origin of ores at Sudbury, it was The key difference, therefore, between endo- tons with 3% Pb, 1.5% Zn, and 70 ppm mal activity in the brecciated carbonates of the suggested that they resulted from the segrega- genie magma with assimilation and the impact Isotopic studies indicate that ore formation is central uplifts. Although individual fragments tion of sulfides as an immiscible liquid, due to models is that a "disequilibrium" composition related to the impact event (Johansson, 1984). of rock are estimated to contain S-20% at, the assimilation of siliceous rocks by a basaltic (with respect to the expected equilibrium Some of these deposits, e.g., at Roda, were Zn magma, followed by gravitational settling and, crystallization of endogenic silicate melts) was exploited commercially until the last century. for example, Serpent Mound, the known sul- later, fractional crystallization and, in some an original property of the SIC. That the metals Other small deposits of Pb, Zn sulfides, and fides are not abundant enough to form a current cases, remobilization (Naldrett et al., 1982; in the ores, as well as the associated silicates, other minerals are known at Crooked Creek, commercial-grade dephsit. Morrison et al., 1994). In the impact model, the have an original crustal source is indicated by USA (7 km, 320 ± 80 Ma), where both galena In large impact structures, the circulation of situation is essentially the same, except that the the Re-Os isotopic composition of the ores and arid barite were mined in the past (Kilsgaard et hot solutions can continue for considerable original source of the metals is crustal and the Nd-Sm composition of the silicates (Fig. 23) al., 1962). Ph and Zn sulfides occur at Serpent time after the impact event and result in sedi- sulfide immiscibility reflects the fact that the (Dickin et al., 1992). While we acknowledge Mound, Ohio, USA (8 km, <320 Ma) (Hey1 and mentary exhalative mineralization. pris is combination of lithologies melted to form the that many details still are to be determined, Brock, 1962) and at Decaturville, USA (6 km, believed to be the origin of the Zo, Ph, Cu, impact melt (SIC) resulted in a liquid with an terent work at Sudlnn'y generally can be fitted < 300 Ma)(Zimmermann and Amstutz, 1972). As, and An mineralization in the Vermillion

136 R. A. E(;NIEI'E ANl) t' l.. M.9.SA1'I'I.S Tr,luusTlu -u. l.tlr tr -r CI? 411. x, I:i: member at the base of the Onwatin formation at structural traps for hydrocarbons. The (-- 60 Sudbury (Rousell, 1984; Davies et al., 1990). rit) post-impact Oil Creek shale within the Ames These deposits contain over 6 million tons of structure also is the source, of the hydrocarbons 4.4% Zn, 1.4% Cu, and 1.2% Pb, and have been (Kuykendall et al., 1994). The general form of mined in the past (Gibson et al., 1994). The the structure is visible in the variations in depth fine-grained nature of the ore, however, has of the overlying sediments (Fig. 24). caused problems in recovery and the ores cur- The first oil and gas discoveries were in 1990 rently are not exploited (Rousell, 1984). At from an approximately 500-m-thick section of Puchezh-Katunki, Russia (80 km, 175 ± 5 Ma), Lower Ordovician Arbuckle dolomite in the the post-impact sediments contain gritstone rim. Locally, the. Arbuckle does not have much and clays. There is a basin formed over a central matrix porosity, but as a result of impact- pit in the central uplift of the structure, and induced fracturing arid karsting, the Arbuckle here the gritstone has been altered to up to 30% in the rim of Ames has considerable economic zeolite (Masaitis et al., 1980). These deposits potential. For example the 27-4 Cecil well, currently are not exploited. Hydrothermal min- drilled in 1991, had drill stem rates of 3440 eralization of post-impact sediments is to some million cubic feet of gas and 300 barrels of oil degree transitional between syngenetic and epi- per day (Roberts and Sandridge, 1992). Wells genetic, as it requires a thermal component that drilled in this center failed to encounter the is directly related to the impact event. Arbuckle dolomite and bottomed in granite Hydrocarbons breccia of the central uplift or, closer to the rim, granite-dolomite breccia. These wells include Hydrocarbons occur at a number of impact the famous Gregory 1-20, which is reported to structures. Only the major deposits—at the be the most productive oil well from a single pay Ames. Red Wing Creek, and Avak structures— zone in Oklahoma. It encountered a roughly 80- are discussed here. to section of granite breccia below the Oil Creek Ames. The Ames structure is located in shale with very effective porosity. A drill stein Oklahoma, USA, and contains the town of test of the zone flowed at approximately 1300 Ames within its boundaries. It is a complex barrels of oil per day, with a conservative esti- structure, roughly 14 km in diameter, with a mate of primary recovery of in excess of 10 central uplift, an annular trough, and slightly million barrels from this single well (Carpenter, uplifted rim. It is buried by up to 3 km of pers. comm., 1994; Kuykendall et al., 1994). Ordovician to Recent sediments (Carpenter Approximately 100 wells have been drilled, 52 and Carlson, 1992). The structure is a recent of which arc producing oil, 1 of which produces discovery, having been identified during the gas. Rased on the initial two years of produc- course of oil exploration (Roberts and Sand- tion, conservative estimates of reserves at Ames ridge, 1992). suggest they will exceed 50 million barrels of oil A local structure known as the Minton and 60 billion cubic feet of gas, which is ener- graben- in which the Ordovician-Silurian getically equivalent to an additional 40 million Ilunton Formation thickens from ca. 70 m to ca. 145 m and lies roughly 60 m below its regional barrels of oil (Isaac and Stewart, 1993; Carpen- level, has been known for some time. During ter, pers. comm., 1994). recent oil exploration, it was recognized that lice age of the structure is I.owcr Ordovician, ibis was part of a much larger structure and that ca. 450 ± 10 Ma, based on the absence of the this graben corresponds to thickened post- Arbuckle dolomite within the structure and the impact deposits in the annular trough of a presence of the (1i1 (:reek shale over the entire complex impact structure (Fig. 24). The rim of structure. It is believed that the impact was the structure is defined by the struct orally high sufficient to result in emergent topography in Lower Ordovician Arbuckle dolomite, and the rirn area and central uplift, the latter having 600 m of Cambrian-Ordovician strata and some been uplifted a minimum of 600 in. This underlying basement are missing in the center resulted in granite wash around the central of the structure as a result of excavation. The uplift and karst topography in the rim (Roberts entire structure is covered by Middle Ordovi- and Sandridge, 1992). This has further cian Oil Creek shade, which forms a seal for enhanced the porosity ul the reservoir rocks. 38 R. A. F GRIEVE .4NP V L. MA.SAIT IS TERRE.STRIAI. Mi PACT CRATERS 139

lydrocarhon production is from the Arbuckle ventral uplift. The drillhole indicated a struc- TRUE Oa Rn nar Burlington Northern (SE tor Stow, 2?) lolomite, the brecciated granite, and granite. turally high and thickened Mississippian and TO 11110116g River lolomite breccia, and largely is due to impact- Pennsylvanian section, compared to drillholes • • • • nduced fracturing and brecciation, which has outside the structure. The well, however, was a f RIM~FRING DEPRESSION I. CENTRAL UPLIFT .e RING DEPRESSION--►~--RFM--► -esulted in significant porosity and, more dry hole. In 1968, Shell drilled another hole to TOP rM.oT. '----~ Too of BOOM / INNOu .mportantiy, permeability. The source of the the NW in the annular trough. Here, the Mis- I;{ I;1 VI Ames oil is the lower section of post-impact Oil sissippian was found to be structurally low .1202 REMROI. Creek shale, which has not been recognized compared to exterior. It also was a dry hole and outside the feature (Castano et al., 1994). In the the structure, as a whole, was assumed to be dry. case of Ames, the impact produced not only the However, True Oil redrilled what was later rec- .,.0e required structural traps but also the paleo- ognized as the central uplift in 1972 and dis- • environment for the deposition of post-impact covered ca. 820 m of Mississippian oil column, shales that upon subsequent burial and matura- with considerable high-angle structural com- tion provided oil and gas. There are similarities plexity and brecciation and a net pay of ca. 490 between the Ames crater shale and a locally m. This is in contrast to the surrounding area, developed Ordovician shale in the Newporte with gentle dips and roughly 30-m oil columns. (North Dakota) structure, an oil-producing sus- The large oil column is the result of the pected impact crater (— 120,000 barrels per structural repetition of what apparently is the year) in Precambrian basement rocks of the Mississippian Mission Canyon Formation in the • Hydro -carton prodlclna wene Dry wena Williston Basin (Castaiio et ai, 1994). The central uplift (Brenan et al., 1975). The addi- Ames discovery has important implications for tion of impact-induced porosity and per- Ftc. 25. Geological cross-section across the Red Wing Creek crater. Hydrocarbons are produced front the oil and gas exploration in crystalline rock meability results in relatively high flow rates of central uplift area. Source: After Gerhard et al. (1982). underlying petroleum basins. Donofrio (1981) over 1000 barrels per day, with cumulative pro- proposed the existence of such hydrocarbon- duction since discovery in excess of 12.7 mil- time between 95 and 3 Ma. Kirschner et al. occur in situ south and southeast of Avak 1. This bearing impact craters and that major oil and lion barrels of oil and 16.2 billion cubic feet of (1992) suggest that the formation of Avak may breccia provides evidence for centripetal trans- gas deposits may occur in brecciated basement natural gas (Pickard, 1994). Current produc- have triggered submarine landslide deposits in port toward the center of 3 km in the forma- rocks. If these structures also generate condi- tion is restricted to ^-300,000 barrels per year, the Aptian and Albian Formation 200 km to the tion of the central uplift (Kirschner et M.,' tions favorable for source rock development, to preserve unexploited reserves of natural gas. cast. If this is the case, its age is 100 ± 5 Ma. 1992). such as at Ames, their potential could be It is estimated that the brecciated central uplift The structure itself has the form of a complex Oil shows occur in Avak I , but the well is not considerable. contains over 130 million barrels of oil, and impact structure ca. 12 km in diameter (Fig. a commercial producer. Indeed, Kirschrier''et al. - Red Wing Creek. A slightly different situation primary and secondary recoverable reserves 26). It is bounded by listric faults, which define (1992) suggest that pre-Avak hydrocarbon occurs at the Red Wing Creek structure in may exceed 70 million barrels (Donofrio, 1981; a rim area, and has an annular trough and accumulations may have been disrupted and North Dakota, USA, where hydrocarbons also Pickard, 1994). The natural gas reserves are central uplift. In the central uplift, the Lower- lost due to the formation of the Avak Structure. are recovered from the brecciated rocks of the estimated at 100 billion cubic feet. Virtually all Middle Jurassic Kingak Shale and Barrow Sand There are, however, the South Barrow, East central uplift. In this case, the impact structure the oil has been discovered within a diameter of are uplifted more than 500 m from their Barrow, and Sikulik gas fields (Fig. 26), which provides the structural trap but, unlike Ames, is — 3 km, corresponding to the central uplift. regional levels. The central uplift has been are post-impact and are related 'to the Avak not responsible for the source of the oil. lieu Based on net pay and its limited aerial extent, drilled by the Avak 1 well, which penetrated to a structure. They occur outside the structure and Wing Creek is a complex structure, approx- depth of 1225 in. The well penetrated the gen- are due to listric faults of the crater rho', which imately 9 km in diameter, with seismic records Red Wing is probably the most prolific oil field eral succession of the area: Lower Cretaceous have truncated the Lower Jurassic Barrow Sand and drill-core data indicating a central peak in in the United States, with the wells in the pebble shale, Jurassic Kingak Shale, Lower and placed Lower Cretaceous Torok shales which strata have been uplifted up to 1 km, an central uplift having the highest cumulative Jurassic Barrow Sand and (Ordovician) Fran- against the sand, creating an effective up-dip gas annular trough containing crater-fill products, productivity of all the wells in North Dakota. kilian argillite. In addition to structural uplift seal. The South and East Borrow fields cur- and a partially eroded structural rim (Fig. 25) Avak. The Avak structure is located on the and the occurrence of shock metamorphic fea- rently are in production and primary recover- (Brenan et al., 1975; Sawatsky, 1977). The Arctic coastal plain of Alaska. The structure has tures, repetition of beds and out-of-sequence able gas is estimated at 37 billion cubic feet structure is formed in Silurian to Triassic car- been known for some time as a "disturbed beds were encountered (Collins, 1961; (Lantz, 1981). The Sikulik gas field is not cur- bonates with minor sandstones and evaporitcs zone" in seismic data (Lantz, 1981) and only Kirschner et al., 1992). For example, the pebble rently exploited. and is buried by — 1.5 km of Jurassic to Neo- recently was evidence of shock metamorphism shale, which has a regional thickness of 1'10 m, Olier structures. gene post-impact sediments. On this basis, the forthcoming in the form of shatter cones and Other impact stnurtures also has an aggregate thickness of 320 in in the Avak structure is assigned an age of 200 ± 25 Ma planar deformation features in quartz produce hydrocarbons. For example, the 25-km- well, caused by steep dips and repetition by (Gerhard et al., 1982). (Kirschner etal., 1992). Avak is buried by late diameter Steen River structure, Canada, pro- faulting. Breccia containing argillite clasts in a As the result of a pronounced anomaly in Pleistocene and Quaternary deposits and duces a respeectaltle 600 barrels of oil per day seismic data, the structure was drilled in 1965 affects Ordovician:Silurian to Cretaceous matrix of Jurassic. arid Cretaceous rocks also front two wells on the northern ruin. Gas is also by Shell Oil, which drilled the NW flank of the rocks. Thus, its age is only constrained to sonie- was encountered. Included as clasts in some produced at the 22-km-diameter Marquez stru t•- breecias are Triassic and Permian strata, which ture, USA. There may be a zone of potential

Th.RRESTRIAI. IMPACT C1tA7'l: R.ti Ill 4(1 R. A. E GRIEVE AN!) If I.. MA.SAITLS

156°30' - 71'20' CNUKCIII Ttvm Lagoon SEA ---- -1030

-_--- - --- -970 0 2000m 2® 3 4E] 5 6 `{:g 7F- 8 WI 910 ~~ 1 • -850, Ftc. 27. Geological cross-section of the Roltysh. Legend: 1 = Quarternary clays, sands; 2 = Eocene to -790---- Paleocene clays, siltstones; 3 = Paleocene-Upper Cretaceous oil shales, siltstones; 4 = Lower Cretaceous sandstones and carbonates; 5 = impact melt rocks; 6 = afloat {moons breccia; 7 = Proterozoic granites, a st Bartow Gas Fie to gneisses; 8 = fractured and brecciated gneisses. South Barre Gas Field noted (e.g., Gudlaugsson, 1993; Isaac and Stew- suggests that they originateld from the erosion art, 1993; Tooth and Stewart, 1994). of ejects deposits and later reworking. A similar -730 ~ situation occurs with respect to the moldavite t r Oil shales tektite placers in Bohemia, which have. Ries as ~ tl---- 790 Oil shales are known at Boltysh (25 km, 88 ± their original source (Botiska et al., 1973): The 3 Ma), Obolon (15 km, 215 ± 25 Ma), and -190 - moldavites are used as stones in the production -g5o / Rotmistrovka (2.7 km, 140±20 Ma) in Ukraine of jewelry (Turnovec, 1987; Turnovec and Sev- __L-IH 9 --L -e- (Masaitis et al., 1980; Gurov and Gurova, cik, 1987). Jewelry also is produced locally from AU LTi 1991). They represent the unmatured equiv- the tektite-like glasses, known as irghizites, -71°10' alent of the hydrocarbon reserves at Ames. The `-~-' found within the Zliamanshin structure (13.5 i ~--I-• f • n most significant reserves are at Bolt ysh, where Fault Antiform Synform Town Wells km 0.9 ± 0.1 Ma), Kazaksthan (Schnetzler, there is an estimated 4.5 billion metric tons. pers. comm., 1994). FIG. 26. Structure contour map on top of Franklinian argillite (Ordovician-Silurian) at Avak. Contour The Boltysh was formed in Archean and Lower interval 60 m. Source: After Kirschner et al. (192). Gas fields are indicated. Proterozoic granites and is completely buried Other deposits by Upper Mesozoic and Cenozoic sediments The post-impact sedimentary fill at the Ries dratigraphic trapping within the 55-km-diarne- been reported in the upper portion of these (Fig. 27). It was recognized initially as a geo- crater, Germany (24 km, 15.1' ± 1.0 Ma), con- er Tookoonooka structure, Australia. More breccias and their genesis has been linked to the physical anomaly and has been drilled exten- tains beds of lignite and bentonite; deposits of niportantly for hydrocarbon exploration, how- 180.krn Chicxulub crater in nearby Yucatan, sively. Surrounding the central uplift is • an diatomite occur at Ragozinka, Russia ,(9 km, 55 ;ver, the Tookoonooka structure has created a Mexico (Limon et al., 1994), which is at the annular trough partially filled by a coherent ± 5 Ma); and phosphorite at I.ogoisk, Belarus ihadow zone to hydrocarbon migration from the center of the debate as to whether its formation impact melt sheet, suevites, and allochthonous (17 km, 40 ± 5 Ma). Gypsum and anhydrite i,romanga Basin depocenter since the early Cre- was or was not the cause of the Cretaceous- breccia. They are capped by various shales, from the Jurassic-aged- post-impact sediments aceous (Gorter et al., 1989). A number of other Tertiary mass extinction event (Hildebrand et including oil shales, and siltstones (Fig. 28). have been quarried since 1901 at Saint Martin, mpact structures have been drilled in the al., 1991). There also are a number of struc- The total productive sequence is ca. 400-500 m Canada (40 km, 219.5 ± 32-Ma). Other local nurse of hydrocarbon exploration-e.g., tures that may be of impact origin, such as and consists of five sections containing 25-40 concentrations of gypsum, anhydrite, and other tosses Bluff, Australia-and generally before Newporte, I ISA, which was rioted earlier (Clem- beds of oil shale with thicknesses up to 4.5 ni chemical sediments are known at other impact hey were recognized as impact structures. On ent and Mayhew, 1979), and Viewfield, Canada (Bass et al., 1967). The oil shales are the result structures and owe their origin to )cost-impact he other hand, Siljan represents a case where (Sawat sky, 1977), that produce hydrocarbons. of biological activity involving algae in this chemical sedimentation from saline lakes. At iydrocarbon exploration was undertaken, in In these two cases, the impact interpretation isolated basin. They currently are not exploited. present, various carbonates, bicarbonates, and -tart, because it was an impact structure. In this suggests they are simple howl-shaped craters. In Placers chlorides are exploited at Lonar, India (1.8 km, neither case have shock metamorphic effects ;asc, exploration, which was highly controver- 0.05 ± 0.01 Ma) and Saltpan, South Africa (1.1 been identified. The recoverable reserves asso- Placer deposits fall into the category of epi- ;ial, was aimed at what were believed to be km, 0.22 f 0.05 Ma), both from saline. lakes, ciated with Viewfield are estimated to be 20 genetic deposits. These result from terrestrial ibiogenic hydrocarbons from- the mantle that resulting from a local closed catchrnent basin million barrels of oil (Isaac and Stewart, 1993). erosion of impact lithologies and include placer were channelled to the Siljan area because of and a warm climate. mpact-induced fracturing of the crust (Gold, Over 500,000 barrels of oil have been produced diamonds related to Popigay, found up to dis- tances of 150 km from the structure, and other 1988). since 1978 from the Calvin structure, USA, which most likely is an 8.5-km-diameter com- occurrences of what arc believed to he impact- Oil from Jurassic rocks is recovered from the Concluding Remarks related diamonds in Ukraine, Kazakhstan, and Lomas Triste breccia deposit at the Cretaccous- plex impact structure (Milstein, 1988). In addi- the Urals of Russia (Masaitis, I993a). The In this review, we have attempted to illustrate lèrtiary boundary in the Campeche oil field in tion, a number of seismic targets with the occurrence of impact-related diamonds at con- the known scope of potential economic deposits the Gulf of Mexico (Camargo Zanoguera and ,'llaracteristics of impact structures, which siderable distances from their source crater in impact structures. They vary from minor (<)uezada Muneton, 1992). Shocked quartz has have hydrocarbon potential, also have been 142 R. .4. E GRIEVE AN!) C I,. MAS.4fT1S TERRFSTR1Af, IMPACT CRATERS 143

land surface in the last 2 Ca is approximately and are, thus, more likely to be concentrated in Bass, Yu. R., Calaka, A. 1., and Grabovskiy, V. f.. 1967, . a 10. At present 3 are known: Chicxuhtb, Sud- and around such lithologies as impact melt The Boltysh oil shales. in Exploration and conselrva i'(1 4.r3 p clays, sands lion of minerals: p. I I-IS (in Russian). E • bury, and Vredefort. Chir.xulub is buried sheets and suevitic hreccias. Secondary epr! Bell, K., Cacriotti, A. I)., and Schnessl, 1. H., 1985, beneath roughly one kilometer of post-impact genetic associations also arc to be expected as a Petrography and geochemistry of the Earl Rivçr sediments and is unlikely, therefore, to have result of hydrothermal processes in areas above Complex, Carswell structure, Saskatchewan—a pos- 80 • :ô.;,,: - clays, silt economically viable mineral deposits, although and below such lithologies. The brecciated and it has exerted considerable control over the sible Proterozoic . Komatütie succession: Geol. 1 fractured rocks of the central uplift also provide Assoc. Canada, Spec. Paper 29, p. 71-80. local hydrology (Pope et al., 1991) and may be oil shales, silt, clays an environment for the structural repetition of Bottomley, R. J., York, D., and Grieve, R. A. F. 1990, the cause of the hydrocarbon-bearing Lomas beds and increased porosity and permeability 40 Argon-19 Argon dating of impact craters labs,!: 5 50 clay marls, oil shales

3 Triste breccia deposit. Both Sudbury and Vre-

I and would appear to be targets for hydrocarbon Proc. 20th Lunar Planet. Sci. Conf., p. 421-431. defort are associated with major mining camps 20 oil shales exploration. Other epigenetic deposits are most Bouska, V., Benada, J., Randa. Z., and Kuncir, 1., 1973, with world-class syngenetic and progenetic ore closely linked with the crater form itself, gener- Geochemical evidence for the origin of moldavites: deposits, respectively. It would appear that the Geochim. et Cosmochim. Acta, v. 37, p. 121-132.

ally as an isolated basin with localized sedimen- otttam largest impact structures have the higher proba- limestones, siltstones, tary and geochemical activity not present in the Brenan, R. L., Peterson, B. L., and Smith, H: 1., 1975, 1111 clay shales, oil shales bility of having significant economic resources. The origin of Red Wing Creek Structure: McKenzie 210m area as a whole or as a barrier to the migration of This is not surprising, as they are the most County, North Dakota: Wyoming Geol .'Assoc.. Earth external fluids. The fact that these isolated r energetic events, affect the largest volumes of Sci. Bull., no. 8, p. I-41. target rocks, have the largest post-impact hydro- basins within impact craters not only can pro- Ruchwald, V. F., 1975. Handbook of iron meteorites: vide the structural traps but also can produce ~ii iiii iii impact lithologies thermal systems, and form the largest topo- Berkeley, Univ. California Press, p. 937-942. graphic basins. the source rocks for hydrocarbons, such as at Bunch, T. E., Dence, M. R., and Cohen, A.'1., 1967, Fa:. 28. Stratigraphie column of post-impact sediments, Impact structures do have a general property Ames, enhances considerably their exploration Natural terrestrial maskelynitei'Amer. Mineral., v.' containing oil shales, at Roltysh. that is an advantage in exploitation. At a large prospects, for they need not occur in known 52, p. 244-253. ' scale, they tend to have fixed morphometric and hydrocarbon-producing areas to have hydrocar- Camargo Zanoguera, A., and Quezada Maneton, 1., deposits of little commercial value to world- structural relationships for a given diameter. bon potential. 1992, Analysis of economic geology of areas of thé class orebodies and significant hydrocarbon The advantage is that once a structure is known Gulf of Mexico with hydrocarbon potential: Rolettn producers. Given the relatively small number of to be of impact origin and its diameter is known, Asoriaciôn Géologos Pririderos, v. XLI, p: I-32 din Spanish). known impact structures, it would seem that, as it is possible to make considerable predictions Acknowledgments a class of geologic features, impact structures as to the structural and lithological character of Carpenter, R. N., and Carlson, R., 1992, The Ames This work was initiated while RAFC wa8 impact crater: Oklahoma Geol. Survey, y. 52, p. have considerable economic potential overall. the feature as a whole. This is generally not For example, the total gross direct worth of possible for most endogenic geologic features. an Alexander von Humboldt-Stiftung For: 208-223. Carter, N. I.., 1965, Basal quartz deformation lamellae— materials extracted from impact structures in The development of an exploitation strategy schungspreistri ger at the Institut für Plan- a criterion for recognit ioni of impactites: Amer. Jour. North America is estimated at f5-6 billion per based on these relationships, once a particular etologie, Munster, receipt of which is gratefully Sci., v. 263, p. 786-806. , year. As noted in the introduction, this type of structure is recognized as being of impact ori- acknowledged. D. Donofrio and A. Therriault , 1968, Dynamic, deformation of quartz, in accounting does not include other economic gin, is most notably illustrated by the drilling read an earlier version of the manuscript and French, R. M., and Short, N, M-, cds.,, Shock meta• considerations, such as impact structures as for hydrocarbons at Ames. One can only won- their comments have helped greatly, as has a morphism of natural materials: Baltimore, Mono current sources for drinking or mineralized der how Sudbury would have been explored and formal review by R. Hargraves. J. H. Stark, Book Corp., p. 453-474. water (e.g., Manson, USA; Kaluga, Russia), as exploited had our current state of knowledge of Continental Resources, kindly supplied the Castatio, J. It., Clement, J. It.. Kuykendall, M. D., and lakes for the generation of hydroelectric power its impact origin and tectonic evolution been original version of Figure 24, depicting the Sharpton, V. I.., 1994, Source rock potential of known soon after it had been discovered. For the (Manicouagan, Canada; Puchezh-Katunki, Ames structure. impact craters labs.!: AAPC Ann. Convention, p.. Russia), as sources of building materials (Ries, case of Sudbury, the level of current knowledge 118. Rouchechouart), and the direct and indirect on its origin and evolution undoubtedly will be REFERENCES Chai, C., and Eckstrand, O. R., 1993, Origin of the benefits that accrue from the establishment of used in future exploration. Sudbury Igneous Complex, Ontario—differentiate Ahrens, T. J., and O'Keefe. J. 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